4.9-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jann Horn <jannh@xxxxxxxxxx>
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")
Cc: stable@xxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
mm/mmap.c | 7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2345,12 +2345,11 @@ int expand_downwards(struct vm_area_stru
struct mm_struct *mm = vma->vm_mm;
struct vm_area_struct *prev;
unsigned long gap_addr;
- 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 */
gap_addr = address - stack_guard_gap;
