KASAN: slab-out-of-bounds Read in skb_ensure_writable

KASAN: slab-out-of-bounds Read in skb_ensure_writable

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    0512e0134582 Merge tag 'xfs-4.17-fixes-3' of git://git.ker..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14956af7800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=968b0b23c7854c0b
dashboard link: https://syzkaller.appspot.com/bug?extid=e5190cb881d8660fb1a3
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=123d9d7b800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=100329d7800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+e5190cb881d8660fb1a3@syzkaller.appspotmail.com

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KASAN: slab-out-of-bounds in pskb_may_pull include/linux/skbuff.h:2104  
[inline]
BUG: KASAN: slab-out-of-bounds in skb_ensure_writable+0x554/0x620  
net/core/skbuff.c:5101
Read of size 4 at addr ffff8801aefc1780 by task syz-executor159/4509

CPU: 0 PID: 4509 Comm: syz-executor159 Not tainted 4.17.0-rc7+ #78
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x1b9/0x294 lib/dump_stack.c:113
  print_address_description+0x6c/0x20b mm/kasan/report.c:256
  kasan_report_error mm/kasan/report.c:354 [inline]
  kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
  __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
  pskb_may_pull include/linux/skbuff.h:2104 [inline]
  skb_ensure_writable+0x554/0x620 net/core/skbuff.c:5101
  __bpf_try_make_writable net/core/filter.c:1419 [inline]
  bpf_try_make_writable net/core/filter.c:1425 [inline]
  ____bpf_l3_csum_replace net/core/filter.c:1546 [inline]
  bpf_l3_csum_replace+0x8c/0x4d0 net/core/filter.c:1537

Allocated by task 0:
(stack is not available)

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8801aefc1680
  which belongs to the cache skbuff_head_cache of size 232
The buggy address is located 24 bytes to the right of
  232-byte region [ffff8801aefc1680, ffff8801aefc1768)
The buggy address belongs to the page:
page:ffffea0006bbf040 count:1 mapcount:0 mapping:ffff8801aefc1040 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801aefc1040 0000000000000000 000000010000000c
raw: ffffea0006b61060 ffffea0006bd69e0 ffff8801d9bdd6c0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8801aefc1680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff8801aefc1700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801aefc1780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                    ^
  ffff8801aefc1800: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff8801aefc1880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

Re: KASAN: slab-out-of-bounds Read in skb_ensure_writable

[Daniel Borkmann]

On 06/04/2018 01:36 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    0512e0134582 Merge tag 'xfs-4.17-fixes-3' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14956af7800000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=968b0b23c7854c0b
> dashboard link: https://syzkaller.appspot.com/bug?extid=e5190cb881d8660fb1a3
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=123d9d7b800000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=100329d7800000

#syz fix: bpf: reject passing modified ctx to helper functions