vm: fix vm_pgoff wrap in stack expansion

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 Święcki, using his nasty system call
fuzzer tool.  Good job.

Reported-and-tested-by: Robert Święcki <robert@swiecki.net>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/mm/mmap.c b/mm/mmap.c
index 2ec8eb5a9cd..8c05e5b43b6 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1814,11 +1814,14 @@ 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;
-                        perf_event_mmap(vma);
+                error = -ENOMEM;
+                if (grow <= vma->vm_pgoff) {
+                        error = acct_stack_growth(vma, size, grow);
+                        if (!error) {
+                                vma->vm_start = address;
+                                vma->vm_pgoff -= grow;
+                                perf_event_mmap(vma);
+                        }
                 }
         }
         vma_unlock_anon_vma(vma);