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

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: a16afaf7928b Merge tag 'for-v4.18' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1338f6bf800000
kernel config: https://syzkaller.appspot.com/x/.config?x=314f2150f36c16ca
dashboard link: https://syzkaller.appspot.com/bug?extid=d2d729bdde65dee3eae6
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1173381f800000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=171f90cf800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d2d729...@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_proto_xlat net/core/filter.c:2637
[inline]
BUG: KASAN: slab-out-of-bounds in ____bpf_skb_change_proto
net/core/filter.c:2675 [inline]
BUG: KASAN: slab-out-of-bounds in bpf_skb_change_proto+0xe37/0x1300
net/core/filter.c:2650
Read of size 2 at addr ffff8801b04646c0 by task syz-executor241/4519

CPU: 0 PID: 4519 Comm: syz-executor241 Not tainted 4.17.0+ #93
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_load2_noabort+0x14/0x20 mm/kasan/report.c:431
bpf_skb_proto_xlat net/core/filter.c:2637 [inline]
____bpf_skb_change_proto net/core/filter.c:2675 [inline]
bpf_skb_change_proto+0xe37/0x1300 net/core/filter.c:2650

Allocated by task 0:
(stack is not available)

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8801b04646c0
which belongs to the cache skbuff_head_cache of size 232
The buggy address is located 0 bytes inside of
232-byte region [ffff8801b04646c0, ffff8801b04647a8)
The buggy address belongs to the page:
page:ffffea0006c11900 count:1 mapcount:0 mapping:ffff8801d9a0d080 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffffea0006c49388 ffffea0006ae5c48 ffff8801d9a0d080
raw: 0000000000000000 ffff8801b0464080 000000010000000c 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801b0464580: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801b0464600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801b0464680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff8801b0464700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801b0464780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
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[Daniel Borkmann]

On 06/10/2018 05:27 PM, syzbot wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    a16afaf7928b Merge tag 'for-v4.18' of git://git.kernel.org..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1338f6bf800000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=314f2150f36c16ca
> dashboard link: https://syzkaller.appspot.com/bug?extid=d2d729bdde65dee3eae6
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1173381f800000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=171f90cf800000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+d2d729...@syzkaller.appspotmail.com

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

[Dmitry Vyukov]

On a related note, it seems that it still can unwind past
bpf_skb_change_proto. I think the "net.core.bpf_jit_kallsyms = 1"
sysctl should have been reached syzbot by the time of crash. Are you
sure that's the only thing requires? We are using frame pointer
unwinder just in case.

[Daniel Borkmann]

Nah, I know what happened; just sent a PR fix for syzkaller.

Thanks,
Daniel