mm: pass correct mm when growing stack

Tetsuo Handa reports seeing the WARN_ON(current->mm == NULL) in
security_vm_enough_memory(), when do_execve() is touching the
target mm's stack, to set up its args and environment.

Yes, a UMH_NO_WAIT or UMH_WAIT_PROC call_usermodehelper() spawns
an mm-less kernel thread to do the exec.  And in any case, that
vm_enough_memory check when growing stack ought to be done on the
target mm, not on the execer's mm (though apart from the warning,
it only makes a slight tweak to OVERCOMMIT_NEVER behaviour).

Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1575,7 +1575,7 @@ static int acct_stack_growth(struct vm_area_struct *vma, unsigned long size, uns
 	 * Overcommit..  This must be the final test, as it will
 	 * update security statistics.
 	 */
-	if (security_vm_enough_memory(grow))
+	if (security_vm_enough_memory_mm(mm, grow))
 		return -ENOMEM;
 
 	/* Ok, everything looks good - let it rip */