[v2.6.34-stable 004/165] vm: fix vm_pgoff wrap in stack expansion

[Paul Gortmaker]

From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

                   -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
    If you see a problem with using this for longterm, please comment.
                   -------------------

commit a626ca6a656450e9f4df91d0dda238fff23285f4 upstream.

Commit 982134ba6261 ("mm: avoid wrapping vm_pgoff in mremap()") fixed
the case of a expanding mapping causing vm_pgoff wrapping when you used
mremap.  But there was another case where we expand mappings hiding in
plain sight: the automatic stack expansion.

This fixes that case too.

This one also found by Robert ÅwiÄcki, using his nasty system call
fuzzer tool.  Good job.

Reported-and-tested-by: Robert ÅwiÄcki <robert@xxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[PG: .34 doesn't have perf call via 3af9e859 (in .36); adjust accordingly]
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 mm/mmap.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index b42b469..fe9b76a 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1784,10 +1784,13 @@ static int expand_downwards(struct vm_area_struct *vma,
 		size = vma->vm_end - address;
 		grow = (vma->vm_start - address) >> PAGE_SHIFT;
 
-		error = acct_stack_growth(vma, size, grow);
-		if (!error) {
-			vma->vm_start = address;
-			vma->vm_pgoff -= grow;
+		error = -ENOMEM;
+		if (grow <= vma->vm_pgoff) {
+			error = acct_stack_growth(vma, size, grow);
+			if (!error) {
+				vma->vm_start = address;
+				vma->vm_pgoff -= grow;
+			}
 		}
 	}
 	anon_vma_unlock(vma);
-- 
1.7.12.rc2

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