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mm: enforce min addr even if capable() in expand_downwards()

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: 8869477 ("security: protect from stack expansion into low vm addresses")
Cc: stable@kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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thejh authored and torvalds committed Feb 27, 2019
1 parent 7d762d6 commit 0a1d52994d440e21def1c2174932410b4f2a98a1
Showing with 3 additions and 4 deletions.
  1. +3 −4 mm/mmap.c
@@ -2426,12 +2426,11 @@ int expand_downwards(struct vm_area_struct *vma,
{
struct mm_struct *mm = vma->vm_mm;
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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