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commit 0a1d52994d440e21def1c2174932410b4f2a98a1
parent 7d762d69145a54d169f58e56d6dac57a5508debc
Author: Jann Horn <jannh@google.com>
Date:   Wed, 27 Feb 2019 21:29:52 +0100

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: 8869477a49c3 ("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>

Diffstat:
Mmm/mmap.c | 7+++----
1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/mmap.c b/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;