Documentation/livepatch/livepatch.txt - kernel/quantenna - Git at Google

 =========
 Livepatch
 =========

 This document outlines basic information about kernel livepatching.

 Table of Contents:

 1. Motivation
 2. Kprobes, Ftrace, Livepatching
 3. Consistency model
 4. Livepatch module
    4.1. New functions
    4.2. Metadata
    4.3. Livepatch module handling
 5. Livepatch life-cycle
    5.1. Registration
    5.2. Enabling
    5.3. Disabling
    5.4. Unregistration
 6. Sysfs
 7. Limitations


 1. Motivation
 =============

 There are many situations where users are reluctant to reboot a system. It may
 be because their system is performing complex scientific computations or under
 heavy load during peak usage. In addition to keeping systems up and running,
 users want to also have a stable and secure system. Livepatching gives users
 both by allowing for function calls to be redirected; thus, fixing critical
 functions without a system reboot.


 2. Kprobes, Ftrace, Livepatching
 ================================

 There are multiple mechanisms in the Linux kernel that are directly related
 to redirection of code execution; namely: kernel probes, function tracing,
 and livepatching:

   + The kernel probes are the most generic. The code can be redirected by
     putting a breakpoint instruction instead of any instruction.

   + The function tracer calls the code from a predefined location that is
     close to the function entry point. This location is generated by the
     compiler using the '-pg' gcc option.

   + Livepatching typically needs to redirect the code at the very beginning
     of the function entry before the function parameters or the stack
     are in any way modified.

 All three approaches need to modify the existing code at runtime. Therefore
 they need to be aware of each other and not step over each other's toes.
 Most of these problems are solved by using the dynamic ftrace framework as
 a base. A Kprobe is registered as a ftrace handler when the function entry
 is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
 a live patch is called with the help of a custom ftrace handler. But there are
 some limitations, see below.


 3. Consistency model
 ====================

 Functions are there for a reason. They take some input parameters, get or
 release locks, read, process, and even write some data in a defined way,
 have return values. In other words, each function has a defined semantic.

 Many fixes do not change the semantic of the modified functions. For
 example, they add a NULL pointer or a boundary check, fix a race by adding
 a missing memory barrier, or add some locking around a critical section.
 Most of these changes are self contained and the function presents itself
 the same way to the rest of the system. In this case, the functions might
 be updated independently one by one.

 But there are more complex fixes. For example, a patch might change
 ordering of locking in multiple functions at the same time. Or a patch
 might exchange meaning of some temporary structures and update
 all the relevant functions. In this case, the affected unit
 (thread, whole kernel) need to start using all new versions of
 the functions at the same time. Also the switch must happen only
 when it is safe to do so, e.g. when the affected locks are released
 or no data are stored in the modified structures at the moment.

 The theory about how to apply functions a safe way is rather complex.
 The aim is to define a so-called consistency model. It attempts to define
 conditions when the new implementation could be used so that the system
 stays consistent. The theory is not yet finished. See the discussion at
 http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189

 The current consistency model is very simple. It guarantees that either
 the old or the new function is called. But various functions get redirected
 one by one without any synchronization.

 In other words, the current implementation _never_ modifies the behavior
 in the middle of the call. It is because it does _not_ rewrite the entire
 function in the memory. Instead, the function gets redirected at the
 very beginning. But this redirection is used immediately even when
 some other functions from the same patch have not been redirected yet.

 See also the section "Limitations" below.


 4. Livepatch module
 ===================

 Livepatches are distributed using kernel modules, see
 samples/livepatch/livepatch-sample.c.

 The module includes a new implementation of functions that we want
 to replace. In addition, it defines some structures describing the
 relation between the original and the new implementation. Then there
 is code that makes the kernel start using the new code when the livepatch
 module is loaded. Also there is code that cleans up before the
 livepatch module is removed. All this is explained in more details in
 the next sections.


 4.1. New functions
 ------------------

 New versions of functions are typically just copied from the original
 sources. A good practice is to add a prefix to the names so that they
 can be distinguished from the original ones, e.g. in a backtrace. Also
 they can be declared as static because they are not called directly
 and do not need the global visibility.

 The patch contains only functions that are really modified. But they
 might want to access functions or data from the original source file
 that may only be locally accessible. This can be solved by a special
 relocation section in the generated livepatch module, see
 Documentation/livepatch/module-elf-format.txt for more details.


 4.2. Metadata
 ------------

 The patch is described by several structures that split the information
 into three levels:

   + struct klp_func is defined for each patched function. It describes
     the relation between the original and the new implementation of a
     particular function.

     The structure includes the name, as a string, of the original function.
     The function address is found via kallsyms at runtime.

     Then it includes the address of the new function. It is defined
     directly by assigning the function pointer. Note that the new
     function is typically defined in the same source file.

     As an optional parameter, the symbol position in the kallsyms database can
     be used to disambiguate functions of the same name. This is not the
     absolute position in the database, but rather the order it has been found
     only for a particular object ( vmlinux or a kernel module ). Note that
     kallsyms allows for searching symbols according to the object name.

   + struct klp_object defines an array of patched functions (struct
     klp_func) in the same object. Where the object is either vmlinux
     (NULL) or a module name.

     The structure helps to group and handle functions for each object
     together. Note that patched modules might be loaded later than
     the patch itself and the relevant functions might be patched
     only when they are available.


   + struct klp_patch defines an array of patched objects (struct
     klp_object).

     This structure handles all patched functions consistently and eventually,
     synchronously. The whole patch is applied only when all patched
     symbols are found. The only exception are symbols from objects
     (kernel modules) that have not been loaded yet. Also if a more complex
     consistency model is supported then a selected unit (thread,
     kernel as a whole) will see the new code from the entire patch
     only when it is in a safe state.


 4.3. Livepatch module handling
 ------------------------------

 The usual behavior is that the new functions will get used when
 the livepatch module is loaded. For this, the module init() function
 has to register the patch (struct klp_patch) and enable it. See the
 section "Livepatch life-cycle" below for more details about these
 two operations.

 Module removal is only safe when there are no users of the underlying
 functions.  The immediate consistency model is not able to detect this;
 therefore livepatch modules cannot be removed. See "Limitations" below.

 5. Livepatch life-cycle
 =======================

 Livepatching defines four basic operations that define the life cycle of each
 live patch: registration, enabling, disabling and unregistration.  There are
 several reasons why it is done this way.

 First, the patch is applied only when all patched symbols for already
 loaded objects are found. The error handling is much easier if this
 check is done before particular functions get redirected.

 Second, the immediate consistency model does not guarantee that anyone is not
 sleeping in the new code after the patch is reverted. This means that the new
 code needs to stay around "forever". If the code is there, one could apply it
 again. Therefore it makes sense to separate the operations that might be done
 once and those that need to be repeated when the patch is enabled (applied)
 again.

 Third, it might take some time until the entire system is migrated
 when a more complex consistency model is used. The patch revert might
 block the livepatch module removal for too long. Therefore it is useful
 to revert the patch using a separate operation that might be called
 explicitly. But it does not make sense to remove all information
 until the livepatch module is really removed.


 5.1. Registration
 -----------------

 Each patch first has to be registered using klp_register_patch(). This makes
 the patch known to the livepatch framework. Also it does some preliminary
 computing and checks.

 In particular, the patch is added into the list of known patches. The
 addresses of the patched functions are found according to their names.
 The special relocations, mentioned in the section "New functions", are
 applied. The relevant entries are created under
 /sys/kernel/livepatch/<name>. The patch is rejected when any operation
 fails.


 5.2. Enabling
 -------------

 Registered patches might be enabled either by calling klp_enable_patch() or
 by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
 start using the new implementation of the patched functions at this stage.

 In particular, if an original function is patched for the first time, a
 function specific struct klp_ops is created and an universal ftrace handler
 is registered.

 Functions might be patched multiple times. The ftrace handler is registered
 only once for the given function. Further patches just add an entry to the
 list (see field `func_stack`) of the struct klp_ops. The last added
 entry is chosen by the ftrace handler and becomes the active function
 replacement.

 Note that the patches might be enabled in a different order than they were
 registered.


 5.3. Disabling
 --------------

 Enabled patches might get disabled either by calling klp_disable_patch() or
 by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
 either the code from the previously enabled patch or even the original
 code gets used.

 Here all the functions (struct klp_func) associated with the to-be-disabled
 patch are removed from the corresponding struct klp_ops. The ftrace handler
 is unregistered and the struct klp_ops is freed when the func_stack list
 becomes empty.

 Patches must be disabled in exactly the reverse order in which they were
 enabled. It makes the problem and the implementation much easier.


 5.4. Unregistration
 -------------------

 Disabled patches might be unregistered by calling klp_unregister_patch().
 This can be done only when the patch is disabled and the code is no longer
 used. It must be called before the livepatch module gets unloaded.

 At this stage, all the relevant sys-fs entries are removed and the patch
 is removed from the list of known patches.


 6. Sysfs
 ========

 Information about the registered patches can be found under
 /sys/kernel/livepatch. The patches could be enabled and disabled
 by writing there.

 See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.


 7. Limitations
 ==============

 The current Livepatch implementation has several limitations:


   + The patch must not change the semantic of the patched functions.

     The current implementation guarantees only that either the old
     or the new function is called. The functions are patched one
     by one. It means that the patch must _not_ change the semantic
     of the function.


   + Data structures can not be patched.

     There is no support to version data structures or anyhow migrate
     one structure into another. Also the simple consistency model does
     not allow to switch more functions atomically.

     Once there is more complex consistency mode, it will be possible to
     use some workarounds. For example, it will be possible to use a hole
     for a new member because the data structure is aligned. Or it will
     be possible to use an existing member for something else.

     There are no plans to add more generic support for modified structures
     at the moment.


   + Only functions that can be traced could be patched.

     Livepatch is based on the dynamic ftrace. In particular, functions
     implementing ftrace or the livepatch ftrace handler could not be
     patched. Otherwise, the code would end up in an infinite loop. A
     potential mistake is prevented by marking the problematic functions
     by "notrace".


   + Anything inlined into __schedule() can not be patched.

     The switch_to macro is inlined into __schedule(). It switches the
     context between two processes in the middle of the macro. It does
     not save RIP in x86_64 version (contrary to 32-bit version). Instead,
     the currently used __schedule()/switch_to() handles both processes.

     Now, let's have two different tasks. One calls the original
     __schedule(), its registers are stored in a defined order and it
     goes to sleep in the switch_to macro and some other task is restored
     using the original __schedule(). Then there is the second task which
     calls patched__schedule(), it goes to sleep there and the first task
     is picked by the patched__schedule(). Its RSP is restored and now
     the registers should be restored as well. But the order is different
     in the new patched__schedule(), so...

     There is work in progress to remove this limitation.


   + Livepatch modules can not be removed.

     The current implementation just redirects the functions at the very
     beginning. It does not check if the functions are in use. In other
     words, it knows when the functions get called but it does not
     know when the functions return. Therefore it can not decide when
     the livepatch module can be safely removed.

     This will get most likely solved once a more complex consistency model
     is supported. The idea is that a safe state for patching should also
     mean a safe state for removing the patch.

     Note that the patch itself might get disabled by writing zero
     to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
     code will not longer get called. But it does not guarantee
     that anyone is not sleeping anywhere in the new code.


   + Livepatch works reliably only when the dynamic ftrace is located at
     the very beginning of the function.

     The function need to be redirected before the stack or the function
     parameters are modified in any way. For example, livepatch requires
     using -fentry gcc compiler option on x86_64.

     One exception is the PPC port. It uses relative addressing and TOC.
     Each function has to handle TOC and save LR before it could call
     the ftrace handler. This operation has to be reverted on return.
     Fortunately, the generic ftrace code has the same problem and all
     this is is handled on the ftrace level.


   + Kretprobes using the ftrace framework conflict with the patched
     functions.

     Both kretprobes and livepatches use a ftrace handler that modifies
     the return address. The first user wins. Either the probe or the patch
     is rejected when the handler is already in use by the other.


   + Kprobes in the original function are ignored when the code is
     redirected to the new implementation.

     There is a work in progress to add warnings about this situation.
	=========
	Livepatch
	=========

	This document outlines basic information about kernel livepatching.

	Table of Contents:

	1. Motivation
	2. Kprobes, Ftrace, Livepatching
	3. Consistency model
	4. Livepatch module
	4.1. New functions
	4.2. Metadata
	4.3. Livepatch module handling
	5. Livepatch life-cycle
	5.1. Registration
	5.2. Enabling
	5.3. Disabling
	5.4. Unregistration
	6. Sysfs
	7. Limitations


	1. Motivation
	=============

	There are many situations where users are reluctant to reboot a system. It may
	be because their system is performing complex scientific computations or under
	heavy load during peak usage. In addition to keeping systems up and running,
	users want to also have a stable and secure system. Livepatching gives users
	both by allowing for function calls to be redirected; thus, fixing critical
	functions without a system reboot.


	2. Kprobes, Ftrace, Livepatching
	================================

	There are multiple mechanisms in the Linux kernel that are directly related
	to redirection of code execution; namely: kernel probes, function tracing,
	and livepatching:

	+ The kernel probes are the most generic. The code can be redirected by
	putting a breakpoint instruction instead of any instruction.

	+ The function tracer calls the code from a predefined location that is
	close to the function entry point. This location is generated by the
	compiler using the '-pg' gcc option.

	+ Livepatching typically needs to redirect the code at the very beginning
	of the function entry before the function parameters or the stack
	are in any way modified.

	All three approaches need to modify the existing code at runtime. Therefore
	they need to be aware of each other and not step over each other's toes.
	Most of these problems are solved by using the dynamic ftrace framework as
	a base. A Kprobe is registered as a ftrace handler when the function entry
	is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from
	a live patch is called with the help of a custom ftrace handler. But there are
	some limitations, see below.


	3. Consistency model
	====================

	Functions are there for a reason. They take some input parameters, get or
	release locks, read, process, and even write some data in a defined way,
	have return values. In other words, each function has a defined semantic.

	Many fixes do not change the semantic of the modified functions. For
	example, they add a NULL pointer or a boundary check, fix a race by adding
	a missing memory barrier, or add some locking around a critical section.
	Most of these changes are self contained and the function presents itself
	the same way to the rest of the system. In this case, the functions might
	be updated independently one by one.

	But there are more complex fixes. For example, a patch might change
	ordering of locking in multiple functions at the same time. Or a patch
	might exchange meaning of some temporary structures and update
	all the relevant functions. In this case, the affected unit
	(thread, whole kernel) need to start using all new versions of
	the functions at the same time. Also the switch must happen only
	when it is safe to do so, e.g. when the affected locks are released
	or no data are stored in the modified structures at the moment.

	The theory about how to apply functions a safe way is rather complex.
	The aim is to define a so-called consistency model. It attempts to define
	conditions when the new implementation could be used so that the system
	stays consistent. The theory is not yet finished. See the discussion at
	http://thread.gmane.org/gmane.linux.kernel/1823033/focus=1828189

	The current consistency model is very simple. It guarantees that either
	the old or the new function is called. But various functions get redirected
	one by one without any synchronization.

	In other words, the current implementation _never_ modifies the behavior
	in the middle of the call. It is because it does _not_ rewrite the entire
	function in the memory. Instead, the function gets redirected at the
	very beginning. But this redirection is used immediately even when
	some other functions from the same patch have not been redirected yet.

	See also the section "Limitations" below.


	4. Livepatch module
	===================

	Livepatches are distributed using kernel modules, see
	samples/livepatch/livepatch-sample.c.

	The module includes a new implementation of functions that we want
	to replace. In addition, it defines some structures describing the
	relation between the original and the new implementation. Then there
	is code that makes the kernel start using the new code when the livepatch
	module is loaded. Also there is code that cleans up before the
	livepatch module is removed. All this is explained in more details in
	the next sections.


	4.1. New functions
	------------------

	New versions of functions are typically just copied from the original
	sources. A good practice is to add a prefix to the names so that they
	can be distinguished from the original ones, e.g. in a backtrace. Also
	they can be declared as static because they are not called directly
	and do not need the global visibility.

	The patch contains only functions that are really modified. But they
	might want to access functions or data from the original source file
	that may only be locally accessible. This can be solved by a special
	relocation section in the generated livepatch module, see
	Documentation/livepatch/module-elf-format.txt for more details.


	4.2. Metadata
	------------

	The patch is described by several structures that split the information
	into three levels:

	+ struct klp_func is defined for each patched function. It describes
	the relation between the original and the new implementation of a
	particular function.

	The structure includes the name, as a string, of the original function.
	The function address is found via kallsyms at runtime.

	Then it includes the address of the new function. It is defined
	directly by assigning the function pointer. Note that the new
	function is typically defined in the same source file.

	As an optional parameter, the symbol position in the kallsyms database can
	be used to disambiguate functions of the same name. This is not the
	absolute position in the database, but rather the order it has been found
	only for a particular object ( vmlinux or a kernel module ). Note that
	kallsyms allows for searching symbols according to the object name.

	+ struct klp_object defines an array of patched functions (struct
	klp_func) in the same object. Where the object is either vmlinux
	(NULL) or a module name.

	The structure helps to group and handle functions for each object
	together. Note that patched modules might be loaded later than
	the patch itself and the relevant functions might be patched
	only when they are available.


	+ struct klp_patch defines an array of patched objects (struct
	klp_object).

	This structure handles all patched functions consistently and eventually,
	synchronously. The whole patch is applied only when all patched
	symbols are found. The only exception are symbols from objects
	(kernel modules) that have not been loaded yet. Also if a more complex
	consistency model is supported then a selected unit (thread,
	kernel as a whole) will see the new code from the entire patch
	only when it is in a safe state.


	4.3. Livepatch module handling
	------------------------------

	The usual behavior is that the new functions will get used when
	the livepatch module is loaded. For this, the module init() function
	has to register the patch (struct klp_patch) and enable it. See the
	section "Livepatch life-cycle" below for more details about these
	two operations.

	Module removal is only safe when there are no users of the underlying
	functions. The immediate consistency model is not able to detect this;
	therefore livepatch modules cannot be removed. See "Limitations" below.

	5. Livepatch life-cycle
	=======================

	Livepatching defines four basic operations that define the life cycle of each
	live patch: registration, enabling, disabling and unregistration. There are
	several reasons why it is done this way.

	First, the patch is applied only when all patched symbols for already
	loaded objects are found. The error handling is much easier if this
	check is done before particular functions get redirected.

	Second, the immediate consistency model does not guarantee that anyone is not
	sleeping in the new code after the patch is reverted. This means that the new
	code needs to stay around "forever". If the code is there, one could apply it
	again. Therefore it makes sense to separate the operations that might be done
	once and those that need to be repeated when the patch is enabled (applied)
	again.

	Third, it might take some time until the entire system is migrated
	when a more complex consistency model is used. The patch revert might
	block the livepatch module removal for too long. Therefore it is useful
	to revert the patch using a separate operation that might be called
	explicitly. But it does not make sense to remove all information
	until the livepatch module is really removed.


	5.1. Registration
	-----------------

	Each patch first has to be registered using klp_register_patch(). This makes
	the patch known to the livepatch framework. Also it does some preliminary
	computing and checks.

	In particular, the patch is added into the list of known patches. The
	addresses of the patched functions are found according to their names.
	The special relocations, mentioned in the section "New functions", are
	applied. The relevant entries are created under
	/sys/kernel/livepatch/<name>. The patch is rejected when any operation
	fails.


	5.2. Enabling
	-------------

	Registered patches might be enabled either by calling klp_enable_patch() or
	by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will
	start using the new implementation of the patched functions at this stage.

	In particular, if an original function is patched for the first time, a
	function specific struct klp_ops is created and an universal ftrace handler
	is registered.

	Functions might be patched multiple times. The ftrace handler is registered
	only once for the given function. Further patches just add an entry to the
	list (see field `func_stack`) of the struct klp_ops. The last added
	entry is chosen by the ftrace handler and becomes the active function
	replacement.

	Note that the patches might be enabled in a different order than they were
	registered.


	5.3. Disabling
	--------------

	Enabled patches might get disabled either by calling klp_disable_patch() or
	by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage
	either the code from the previously enabled patch or even the original
	code gets used.

	Here all the functions (struct klp_func) associated with the to-be-disabled
	patch are removed from the corresponding struct klp_ops. The ftrace handler
	is unregistered and the struct klp_ops is freed when the func_stack list
	becomes empty.

	Patches must be disabled in exactly the reverse order in which they were
	enabled. It makes the problem and the implementation much easier.


	5.4. Unregistration
	-------------------

	Disabled patches might be unregistered by calling klp_unregister_patch().
	This can be done only when the patch is disabled and the code is no longer
	used. It must be called before the livepatch module gets unloaded.

	At this stage, all the relevant sys-fs entries are removed and the patch
	is removed from the list of known patches.


	6. Sysfs
	========

	Information about the registered patches can be found under
	/sys/kernel/livepatch. The patches could be enabled and disabled
	by writing there.

	See Documentation/ABI/testing/sysfs-kernel-livepatch for more details.


	7. Limitations
	==============

	The current Livepatch implementation has several limitations:


	+ The patch must not change the semantic of the patched functions.

	The current implementation guarantees only that either the old
	or the new function is called. The functions are patched one
	by one. It means that the patch must _not_ change the semantic
	of the function.


	+ Data structures can not be patched.

	There is no support to version data structures or anyhow migrate
	one structure into another. Also the simple consistency model does
	not allow to switch more functions atomically.

	Once there is more complex consistency mode, it will be possible to
	use some workarounds. For example, it will be possible to use a hole
	for a new member because the data structure is aligned. Or it will
	be possible to use an existing member for something else.

	There are no plans to add more generic support for modified structures
	at the moment.


	+ Only functions that can be traced could be patched.

	Livepatch is based on the dynamic ftrace. In particular, functions
	implementing ftrace or the livepatch ftrace handler could not be
	patched. Otherwise, the code would end up in an infinite loop. A
	potential mistake is prevented by marking the problematic functions
	by "notrace".


	+ Anything inlined into __schedule() can not be patched.

	The switch_to macro is inlined into __schedule(). It switches the
	context between two processes in the middle of the macro. It does
	not save RIP in x86_64 version (contrary to 32-bit version). Instead,
	the currently used __schedule()/switch_to() handles both processes.

	Now, let's have two different tasks. One calls the original
	__schedule(), its registers are stored in a defined order and it
	goes to sleep in the switch_to macro and some other task is restored
	using the original __schedule(). Then there is the second task which
	calls patched__schedule(), it goes to sleep there and the first task
	is picked by the patched__schedule(). Its RSP is restored and now
	the registers should be restored as well. But the order is different
	in the new patched__schedule(), so...

	There is work in progress to remove this limitation.


	+ Livepatch modules can not be removed.

	The current implementation just redirects the functions at the very
	beginning. It does not check if the functions are in use. In other
	words, it knows when the functions get called but it does not
	know when the functions return. Therefore it can not decide when
	the livepatch module can be safely removed.

	This will get most likely solved once a more complex consistency model
	is supported. The idea is that a safe state for patching should also
	mean a safe state for removing the patch.

	Note that the patch itself might get disabled by writing zero
	to /sys/kernel/livepatch/<patch>/enabled. It causes that the new
	code will not longer get called. But it does not guarantee
	that anyone is not sleeping anywhere in the new code.


	+ Livepatch works reliably only when the dynamic ftrace is located at
	the very beginning of the function.

	The function need to be redirected before the stack or the function
	parameters are modified in any way. For example, livepatch requires
	using -fentry gcc compiler option on x86_64.

	One exception is the PPC port. It uses relative addressing and TOC.
	Each function has to handle TOC and save LR before it could call
	the ftrace handler. This operation has to be reverted on return.
	Fortunately, the generic ftrace code has the same problem and all
	this is is handled on the ftrace level.


	+ Kretprobes using the ftrace framework conflict with the patched
	functions.

	Both kretprobes and livepatches use a ftrace handler that modifies
	the return address. The first user wins. Either the probe or the patch
	is rejected when the handler is already in use by the other.


	+ Kprobes in the original function are ignored when the code is
	redirected to the new implementation.

	There is a work in progress to add warnings about this situation.