fs: exec: apply CLOEXEC before changing dumpable task flags
commit 613cc2b6f272c1a8ad33aefa21cad77af23139f7 upstream.
If you have a process that has set itself to be non-dumpable, and it
then undergoes exec(2), any CLOEXEC file descriptors it has open are
"exposed" during a race window between the dumpable flags of the process
being reset for exec(2) and CLOEXEC being applied to the file
descriptors. This can be exploited by a process by attempting to access
/proc/<pid>/fd/... during this window, without requiring CAP_SYS_PTRACE.
The race in question is after set_dumpable has been (for get_link,
though the trace is basically the same for readlink):
-> ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
Which will return 0, during the race window and CLOEXEC file descriptors
will still be open during this window because do_close_on_exec has not
been called yet. As a result, the ordering of these calls should be
reversed to avoid this race window.
This is of particular concern to container runtimes, where joining a
PID namespace with file descriptors referring to the host filesystem
can result in security issues (since PRCTL_SET_DUMPABLE doesn't protect
against access of CLOEXEC file descriptors -- file descriptors which may
reference filesystem objects the container shouldn't have access to).
Cc: <firstname.lastname@example.org> # v3.2+
Reported-by: Michael Crosby <email@example.com>
Signed-off-by: Aleksa Sarai <firstname.lastname@example.org>
Signed-off-by: Al Viro <email@example.com>
Signed-off-by: Greg Kroah-Hartman <firstname.lastname@example.org>
1 file changed