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Subject[PATCH 3.16 181/202] mm: enforce min addr even if capable() in expand_downwards()
3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@google.com>

commit 0a1d52994d440e21def1c2174932410b4f2a98a1 upstream.

security_mmap_addr() does a capability check with current_cred(), but
we can reach this code from contexts like a VFS write handler where
current_cred() must not be used.

This can be abused on systems without SMAP to make NULL pointer
dereferences exploitable again.

Fixes: 8869477a49c3 ("security: protect from stack expansion into low vm addresses")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
mm/mmap.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2244,12 +2244,11 @@ int expand_downwards(struct vm_area_stru
unsigned long address)
{
struct vm_area_struct *prev;
- int error;
+ int error = 0;

address &= PAGE_MASK;
- error = security_mmap_addr(address);
- if (error)
- return error;
+ if (address < mmap_min_addr)
+ return -EPERM;

/* Enforce stack_guard_gap */
prev = vma->vm_prev;
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