CVE-2017-1000365-fs-exec.c-account-for-argv-envp-pointers.patch

   1 From 6b44f156198491839655077fc7f3a469c67e1f8d Mon Sep 17 00:00:00 2001
   2 From: Kees Cook <keescook@chromium.org>
   3 Date: Wed, 12 Jul 2017 15:30:39 +0200
   4 Subject: [PATCH 2/3] fs/exec.c: account for argv/envp pointers
   5
   6 When limiting the argv/envp strings during exec to 1/4 of the stack limit,
   7 the storage of the pointers to the strings was not included.  This means
   8 that an exec with huge numbers of tiny strings could eat 1/4 of the stack
   9 limit in strings and then additional space would be later used by the
  10 pointers to the strings.
  11
  12 For example, on 32-bit with a 8MB stack rlimit, an exec with 1677721
  13 single-byte strings would consume less than 2MB of stack, the max (8MB /
  14 4) amount allowed, but the pointers to the strings would consume the
  15 remaining additional stack space (1677721 * 4 == 6710884).
  16
  17 The result (1677721 + 6710884 == 8388605) would exhaust stack space
  18 entirely.  Controlling this stack exhaustion could result in
  19 pathological behavior in setuid binaries (CVE-2017-1000365).
  20
  21 [akpm@linux-foundation.org: additional commenting from Kees]
  22 Fixes: b6a2fea39318 ("mm: variable length argument support")
  23 Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast
  24 Signed-off-by: Kees Cook <keescook@chromium.org>
  25 Acked-by: Rik van Riel <riel@redhat.com>
  26 Acked-by: Michal Hocko <mhocko@suse.com>
  27 Cc: Alexander Viro <viro@zeniv.linux.org.uk>
  28 Cc: Qualys Security Advisory <qsa@qualys.com>
  29 Cc: <stable@vger.kernel.org>
  30 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  31 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  32
  33 CVE-2017-1000365
  34
  35 (cherry-picked from commit 98da7d08850fb8bdeb395d6368ed15753304aa0c)
  36 Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
  37 Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
  38 Acked-by: Seth Forshee <seth.forshee@canonical.com>
  39 Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
  40
  41 Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
  42 ---
  43  fs/exec.c | 28 ++++++++++++++++++++++++----
  44  1 file changed, 24 insertions(+), 4 deletions(-)
  45
  46 diff --git a/fs/exec.c b/fs/exec.c
  47 index 9fb90c37c37f..8cf76e2a0b83 100644
  48 --- a/fs/exec.c
  49 +++ b/fs/exec.c
  50 @@ -225,8 +225,26 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
  51
  52         if (write) {
  53                 unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
  54 +               unsigned long ptr_size;
  55                 struct rlimit *rlim;
  56
  57 +               /*
  58 +                * Since the stack will hold pointers to the strings, we
  59 +                * must account for them as well.
  60 +                *
  61 +                * The size calculation is the entire vma while each arg page is
  62 +                * built, so each time we get here it's calculating how far it
  63 +                * is currently (rather than each call being just the newly
  64 +                * added size from the arg page).  As a result, we need to
  65 +                * always add the entire size of the pointers, so that on the
  66 +                * last call to get_arg_page() we'll actually have the entire
  67 +                * correct size.
  68 +                */
  69 +               ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
  70 +               if (ptr_size > ULONG_MAX - size)
  71 +                       goto fail;
  72 +               size += ptr_size;
  73 +
  74                 acct_arg_size(bprm, size / PAGE_SIZE);
  75
  76                 /*
  77 @@ -244,13 +262,15 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
  78                  *    to work from.
  79                  */
  80                 rlim = current->signal->rlim;
  81 -               if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) {
  82 -                       put_page(page);
  83 -                       return NULL;
  84 -               }
  85 +               if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
  86 +                       goto fail;
  87         }
  88
  89         return page;
  90 +
  91 +fail:
  92 +       put_page(page);
  93 +       return NULL;
  94  }
  95
  96  static void put_arg_page(struct page *page)
  97 --
  98 2.11.0
  99