mm: rmap: fix use-after-free in __put_anon_vma

commit 624483f3ea82598ab0f62f1bdb9177f531ab1892 upstream.

While working address sanitizer for kernel I've discovered
use-after-free bug in __put_anon_vma.

For the last anon_vma, anon_vma->root freed before child anon_vma.
Later in anon_vma_free(anon_vma) we are referencing to already freed
anon_vma->root to check rwsem.

This fixes it by freeing the child anon_vma before freeing
anon_vma->root.

Change-Id: Id30e912036bcbe47bafbcd714694c53fdcac60b7
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 65375ce7
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>