Re: KASAN: use-after-free Read in __dev_queue_xmit

[Willem de Bruijn]

On Wed, May 9, 2018 at 3:37 AM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
> On Wed, Jan 03, 2018 at 10:53:14PM -0800, Eric Dumazet wrote:
>> On Wed, 2018-01-03 at 21:13 -0800, Eric Dumazet wrote:
>> > Note: all commands must start from beginning of the line in the email body.
>> >
>> > I guess skb_probe_transport_header() should be hardened to reject malicious
>> > packets given by user space, instead of being gentle.
>>
>> Although bug triggered for this particular repro is in flow dissector
>> :/
>>
>> I will test :
>>
>> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
>> index 15ce300637650e17fcab7e378b20fe7972686d46..544bddf08e13c7f6e47aadc737244c9ba5af56b2 100644
>> --- a/net/core/flow_dissector.c
>> +++ b/net/core/flow_dissector.c
>> @@ -976,8 +976,8 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
>>  out_good:
>>         ret = true;
>>
>> -       key_control->thoff = (u16)nhoff;
>>  out:
>> +       key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
>>         key_basic->n_proto = proto;
>>         key_basic->ip_proto = ip_proto;
>>
>> @@ -985,7 +985,6 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
>>
>>  out_bad:
>>         ret = false;
>> -       key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
>>         goto out;
>>  }
>>  EXPORT_SYMBOL(__skb_flow_dissect);
>
> Fix for this was commit d0c081b49137cd:
>
> #syz fix: flow_dissector: properly cap thoff field
>
> But a crash with the same signature is still occurring, so it should eventually
> get reported again.  C reproducer is here, it works on Linus' tree (commit
> 036db8bd963): https://syzkaller.appspot.com/text?tag=ReproC&x=105b1ae7800000

This appears to be a separate issue.

This reproducer requires a setsockopt SOL_SOCKET/SO_TIMESTAMPING
to trigger the use-after-free. And the freed path also points at a timestamping
skb:

[   31.963619] Freed by task 2672:
[   31.964006]  __kasan_slab_free+0x125/0x170
[   31.964509]  kfree+0x8b/0x1a0
[   31.964875]  skb_free_head+0x6f/0xa0
[   31.965314]  skb_release_data+0x420/0x5a0
[   31.965802]  skb_release_all+0x46/0x60
[   31.966260]  kfree_skb+0x91/0x1c0
[   31.966669]  __skb_complete_tx_timestamp+0x2e9/0x3d0
[   31.967273]  __skb_tstamp_tx+0x3b3/0x620
[   31.967774]  __dev_queue_xmit+0xed5/0x1a20
[   31.968300]  packet_sendmsg+0x36fd/0x5400
[   31.968821]  sock_sendmsg+0xc0/0x100
[   31.969284]  ___sys_sendmsg+0x367/0x880
[   31.969777]  __sys_sendmmsg+0x178/0x410
[   31.970267]  __x64_sys_sendmmsg+0x99/0x100
[   31.970789]  do_syscall_64+0x9a/0x2c0
[   31.971260]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The data skb itself is zero bytes, it appears.