netdev - Re: general protection fault in tcp_v6

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Date:   Mon, 3 Jun 2019 08:59:01 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     David Ahern <dsahern@...il.com>
Cc:     syzbot <syzbot+5ee26b4e30c45930bd3c@...kaller.appspotmail.com>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: general protection fault in tcp_v6_connect

On Mon, Jun 3, 2019 at 5:29 AM David Ahern <dsahern@...il.com> wrote:
>
> On 6/1/19 12:05 AM, syzbot wrote:
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    f4aa8012 cxgb4: Make t4_get_tp_e2c_map static
> > git tree:       net-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1662cb12a00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=d137eb988ffd93c3
> > dashboard link:
> > https://syzkaller.appspot.com/bug?extid=5ee26b4e30c45930bd3c
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+5ee26b4e30c45930bd3c@...kaller.appspotmail.com
> >
> > kasan: CONFIG_KASAN_INLINE enabled
> > kasan: GPF could be caused by NULL-ptr deref or user memory access
> > general protection fault: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 1 PID: 17324 Comm: syz-executor.5 Not tainted 5.2.0-rc1+ #2
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 01/01/2011
> > RIP: 0010:__read_once_size include/linux/compiler.h:194 [inline]
> > RIP: 0010:rt6_get_cookie include/net/ip6_fib.h:264 [inline]
> > RIP: 0010:ip6_dst_store include/net/ip6_route.h:213 [inline]
> > RIP: 0010:tcp_v6_connect+0xfd0/0x20a0 net/ipv6/tcp_ipv6.c:298
> > Code: 89 e6 e8 83 a2 48 fb 45 84 e4 0f 84 90 09 00 00 e8 35 a1 48 fb 49
> > 8d 7e 70 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02
> > 00 0f 85 57 0e 00 00 4d 8b 66 70 e8 4d 88 35 fb 31 ff 89
> > RSP: 0018:ffff888066547800 EFLAGS: 00010207
> > RAX: dffffc0000000000 RBX: ffff888064e839f0 RCX: ffffc90010e49000
> > RDX: 000000000000002b RSI: ffffffff8628033b RDI: 000000000000015f
> > RBP: ffff888066547980 R08: ffff8880a9412080 R09: ffffed1015d26be0
>
> This one is not so obvious.
>
> The error has to be a bad dst from ip6_dst_lookup_flow called by
> tcp_v6_connect which then is attempted to be stored in the socket via
> ip6_dst_store. ip6_dst_store calls rt6_get_cookie with dst as the
> argument. RDI (first arg for x86) shows 0x15f which is not a valid and
> would cause a fault.
>
> None of the ip6_dst_* functions in net/ipv6/ip6_output.c have changed
> recently (5.2-next definitely but I believe this true for many releases
> prior). Further, all of the FIB lookup functions (called by
> ip6_dst_lookup_flow) always return a non-NULL dst.
>
> If my hunch about the other splat is correct (pcpu corruption) that
> could explain this one: FIB lookup is fine and finds an entry, the entry
> has a pcpu cache entry so it is returned. If the pcpu entry was stomped
> on then it would be invalid and the above would result.


This happened only once so far, so may be a previous silent memory corruption.

This also may be related to "KASAN: user-memory-access Read in
ip6_hold_safe (3)":
https://syzkaller.appspot.com/bug?extid=a5b6e01ec8116d046842
because that one seems to be a race in involved code.
So this one may be a rare incarnation of the other crash.