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

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

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    75d4e704fa8d netdev-FAQ: clarify DaveM's position for stab..
git tree:       bpf-next
console output: https://syzkaller.appspot.com/x/log.txt?x=16bd21af800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a601a80fec461d44
dashboard link: https://syzkaller.appspot.com/bug?extid=567faa843005dda30737
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1039185f800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11e85cff800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+567faa843005dda30737@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 ____bpf_skb_change_head  
net/core/filter.c:2921 [inline]
BUG: KASAN: slab-out-of-bounds in bpf_skb_change_head+0x80c/0x9d0  
net/core/filter.c:2917
Read of size 4 at addr ffff8801d94ea680 by task syz-executor991/4551

CPU: 0 PID: 4551 Comm: syz-executor991 Not tainted 4.17.0-rc7+ #38
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
  ____bpf_skb_change_head net/core/filter.c:2921 [inline]
  bpf_skb_change_head+0x80c/0x9d0 net/core/filter.c:2917

Allocated by task 0:
(stack is not available)

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8801d94ea580
  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 [ffff8801d94ea580, ffff8801d94ea668)
The buggy address belongs to the page:
page:ffffea0007653a80 count:1 mapcount:0 mapping:ffff8801d94ea080 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801d94ea080 0000000000000000 000000010000000c
raw: ffffea0006b2e720 ffffea00076595e0 ffff8801d9450e40 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8801d94ea580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff8801d94ea600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801d94ea680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                    ^
  ffff8801d94ea700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff8801d94ea780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


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

[Daniel Borkmann]

On 06/14/2018 12:17 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following crash on:
> 
> HEAD commit:    75d4e704fa8d netdev-FAQ: clarify DaveM's position for stab..
> git tree:       bpf-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=16bd21af800000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a601a80fec461d44
> dashboard link: https://syzkaller.appspot.com/bug?extid=567faa843005dda30737
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1039185f800000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11e85cff800000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+567faa843005dda30737@syzkaller.appspotmail.com

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