kernel: add panic_on_taint

Analogously to the introduction of panic_on_warn, this patch introduces
a kernel option named panic_on_taint in order to provide a simple and
generic way to stop execution and catch a coredump when the kernel gets
tainted by any given flag.

This is useful for debugging sessions as it avoids having to rebuild the
kernel to explicitly add calls to panic() into the code sites that
introduce the taint flags of interest.

For instance, if one is interested in proceeding with a post-mortem
analysis at the point a given code path is hitting a bad page (i.e.
unaccount_page_cache_page(), or slab_bug()), a coredump can be collected
by rebooting the kernel with 'panic_on_taint=0x20' amended to the
command line.

Another, perhaps less frequent, use for this option would be as a means
for assuring a security policy case where only a subset of taints, or no
single taint (in paranoid mode), is allowed for the running system.  The
optional switch 'nousertaint' is handy in this particular scenario, as
it will avoid userspace induced crashes by writes to sysctl interface
/proc/sys/kernel/tainted causing false positive hits for such policies.

[[email protected]: tweak kernel-parameters.txt wording]

Suggested-by: Qian Cai <[email protected]>
Signed-off-by: Rafael Aquini <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Luis Chamberlain <[email protected]>
Cc: Dave Young <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Adrian Bunk <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Laura Abbott <[email protected]>
Cc: Jeff Mahoney <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Takashi Iwai <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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