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

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Eric Biggers <ebiggers3@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
	Eric Dumazet <edumazet@google.com>,
	syzbot <syzbot+71d74a5406d02057d559@syzkaller.appspotmail.com>,
	alexander.deucher@amd.com,
	Andrey Konovalov <andreyknvl@google.com>,
	Anoob Soman <anoob.soman@citrix.com>,
	chris@chris-wilson.co.uk, David Miller <davem@davemloft.net>,
	"Reshetova, Elena" <elena.reshetova@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Kees Cook <keescook@chromium.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Mike Maloney <maloney@google.com>,
	mchehab@kernel.org, netdev <netdev@vger.kernel.org>,
	"Rosen, Rami" <rami.rosen@intel.com>,
	Sowmini Varadhan <sowmini.varadhan@oracle.com>,
	syzkaller-bugs@googlegroups.com,
	Willem de Bruijn <willemb@google.com>
Subject: Re: KASAN: use-after-free Read in __dev_queue_xmit
Date: Wed, 9 May 2018 12:11:25 -0400	[thread overview]
Message-ID: <CAF=yD-Je0ej2XDcfyEKano4R-og5oGjPvP36X4dSmpYshzBuKA@mail.gmail.com> (raw)
In-Reply-To: <20180509073754.GG711@sol.localdomain>

On Wed, May 9, 2018 at 3:37 AM, Eric Biggers <ebiggers3@gmail.com> 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.