releases/3.18.137/mm-enforce-min-addr-even-if-capable-in-expand_downwards.patch

   1 From 0a1d52994d440e21def1c2174932410b4f2a98a1 Mon Sep 17 00:00:00 2001
   2 From: Jann Horn <jannh@google.com>
   3 Date: Wed, 27 Feb 2019 21:29:52 +0100
   4 Subject: mm: enforce min addr even if capable() in expand_downwards()
   5
   6 From: Jann Horn <jannh@google.com>
   7
   8 commit 0a1d52994d440e21def1c2174932410b4f2a98a1 upstream.
   9
  10 security_mmap_addr() does a capability check with current_cred(), but
  11 we can reach this code from contexts like a VFS write handler where
  12 current_cred() must not be used.
  13
  14 This can be abused on systems without SMAP to make NULL pointer
  15 dereferences exploitable again.
  16
  17 Fixes: 8869477a49c3 ("security: protect from stack expansion into low vm addresses")
  18 Cc: stable@kernel.org
  19 Signed-off-by: Jann Horn <jannh@google.com>
  20 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  21 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  22
  23 ---
  24  mm/mmap.c |    7 +++----
  25  1 file changed, 3 insertions(+), 4 deletions(-)
  26
  27 --- a/mm/mmap.c
  28 +++ b/mm/mmap.c
  29 @@ -2292,12 +2292,11 @@ int expand_downwards(struct vm_area_stru
  30  {
  31         struct vm_area_struct *prev;
  32         unsigned long gap_addr;
  33 -       int error;
  34 +       int error = 0;
  35
  36         address &= PAGE_MASK;
  37 -       error = security_mmap_addr(address);
  38 -       if (error)
  39 -               return error;
  40 +       if (address < mmap_min_addr)
  41 +               return -EPERM;
  42
  43         /* Enforce stack_guard_gap */
  44         gap_addr = address - stack_guard_gap;