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Subject[PATCH 1/3] mmap: protect from stack expantion into low vm addresses
When adding the new security hooks into mmap to enhance protection
against NULL pointers in the kernel I overlooked that a user may be able
to expand the stack all the way down to low addresses. This flaw was
pointed out by a PaX/grsecurity developer and this patch should provide
protection against this missed code path.

Signed-off-by: Eric Paris <>


** Be very careful applying/rediffing this patch. Standard 3 lines of
context from git diff will misapply the first hunk to expand_upwards
instead of properly in expand_downwards. This patch was generated using
-U 4 to make sure it applies in the correct place. **

mm/mmap.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- kernel-1/mm/mmap.c
+++ kernel-2/mm/mmap.c
@@ -1614,17 +1614,21 @@ static inline int expand_downwards(struc
* so that the anon_vma locking is not a noop.
if (unlikely(anon_vma_prepare(vma)))
return -ENOMEM;
+ address &= PAGE_MASK;
+ error = security_file_mmap(0, 0, 0, 0, address, 1);
+ if (error)
+ return error;

* vma->vm_start/vm_end cannot change under us because the caller
* is required to hold the mmap_sem in read mode. We need the
* anon_vma lock to serialize against concurrent expand_stacks.
- address &= PAGE_MASK;
- error = 0;

/* Somebody else might have raced and expanded it already */
if (address < vma->vm_start) {
unsigned long size, grow;

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