get_wchan() is lockless. Task may wakeup at any time and change its own stack,
thus each next stack frame may be overwritten and filled with random stuff.

/proc/$pid/stack interface had been disabled for non-current tasks, see [1]
But 'wchan' still allows to trigger stack frame unwinding on volatile stack.

This patch fixes oops in unwind_frame() by adding stack pointer validation on
each step (as x86 code do), unwind_frame() already checks frame pointer.

Also I've found another report of this oops on stackoverflow (irony).

Link: http://www.spinics.net/lists/arm-kernel/msg110589.html [1]
Link: http://stackoverflow.com/questions/18479894/unwind-frame-cause-a-kernel-paging-error

Cc: <stable@vger.kernel.org>
Signed-off-by: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Unfortunately, I never committed the fix to a nasty oops which can
occur as a result of that commit:

------------[ cut here ]------------
kernel BUG at /home/olof/work/batch/include/linux/mm.h:414!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
Modules linked in:
CPU: 0 PID: 490 Comm: killall5 Not tainted 3.11.0-rc3-00288-gabe0308 #53
task: e90acac0 ti: e9be8000 task.ti: e9be8000
PC is at special_mapping_fault+0xa4/0xc4
LR is at __do_fault+0x68/0x48c

This doesn't show up unless you do quite a bit of testing; a simple
boot test does not do this, so all my nightly tests were passing fine.

The reason for this is that install_special_mapping() expects the
page array to stick around, and as this was only inserting one page
which was stored on the kernel stack, that's why this was blowing up.

Reported-by: Olof Johansson <olof@lixom.net>
Tested-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

If one process calls sys_reboot and that process then stops other
CPUs while those CPUs are within a spin_lock() region we can
potentially encounter a deadlock scenario like below.

CPU 0                   CPU 1
-----                   -----
                        spin_lock(my_lock)
smp_send_stop()
 <send IPI>             handle_IPI()
                         disable_preemption/irqs
                          while(1);
 <PREEMPT>
spin_lock(my_lock) <--- Waits forever

We shouldn't attempt to run any other tasks after we send a stop
IPI to a CPU so disable preemption so that this task runs to
completion. We use local_irq_disable() here for cross-arch
consistency with x86.

Reported-by: Sundarajan Srinivasan <sundaraj@codeaurora.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

If kuser helpers are not provided by the kernel, disable user access to
the vectors page.  With the kuser helpers gone, there is no reason for
this page to be visible to userspace.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Move the signal handlers into a VDSO page rather than keeping them in
the vectors page.  This allows us to place them randomly within this
page, and also map the page at a random location within userspace
further protecting these code fragments from ROP attacks.  The new
VDSO page is also poisoned in the same way as the vector page.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Merge together the unicore32, arm, and x86 reboot= command line
parameter handling.

Signed-off-by: Robin Holt <holt@sgi.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Russ Anderson <rja@sgi.com>
Cc: Robin Holt <holt@sgi.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>