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

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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: 8869477a49 ("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>
This commit is contained in:
Jann Horn 2019-02-27 21:29:52 +01:00 committed by syphyr
parent bf68acc5c9
commit de23c77fdb
1 changed files with 3 additions and 4 deletions
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7
mm/mmap.c
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@ -2256,12 +2256,11 @@ int expand_downwards(struct vm_area_struct *vma,
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;