mm: check for no mmaps in exit_mmap()

commit dcd4a049b9751828c516c59709f3fdf50436df85 upstream.

When dup_mmap() ooms we can end up with mm->mmap == NULL.  The error
path does mmput() and unmap_vmas() gets a NULL vma which it
dereferences.

In exit_mmap() there is nothing to do at all for this case, we can
cancel the callpath right there.

[akpm@linux-foundation.org: add sorely-needed comment]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Kir Kolyshkin <kir@openvz.org>
Tested-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/mm/mmap.c b/mm/mmap.c
index ca12a930843624fe2df3e3265253052116f3d8f6..2ae093ed2cac75f068f2dbbe47518fd2171eebfd 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2068,6 +2068,9 @@ void exit_mmap(struct mm_struct *mm)
        arch_exit_mmap(mm);
        mmu_notifier_release(mm);
 
+       if (!mm->mmap)  /* Can happen if dup_mmap() received an OOM */
+               return;
+
        lru_add_drain();
        flush_cache_mm(mm);
        tlb = tlb_gather_mmu(mm, 1);