Date:   Mon, 1 May 2023 13:19:00 -0700
From:   Kuniyuki Iwashima <kuniyu@...zon.com>
To:     <willemdebruijn.kernel@...il.com>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <kuni1840@...il.com>, <kuniyu@...zon.com>,
        <netdev@...r.kernel.org>, <pabeni@...hat.com>,
        <shaozhengchao@...wei.com>, <syzkaller@...glegroups.com>
Subject: Re: [PATCH v1 net] af_packet: Don't send zero-byte data in packet_sendmsg_spkt().

From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date:   Mon, 01 May 2023 15:59:51 -0400
> Willem de Bruijn wrote:
> > Kuniyuki Iwashima wrote:
> > > syzkaller reported a warning below [0].
> > > 
> > > We can reproduce it by sending 0-byte data from the (AF_PACKET,
> > > SOCK_PACKET) socket via some devices whose dev->hard_header_len
> > > is 0.
> > > 
> > >     struct sockaddr_pkt addr = {
> > >         .spkt_family = AF_PACKET,
> > >         .spkt_device = "tun0",
> > >     };
> > >     int fd;
> > > 
> > >     fd = socket(AF_PACKET, SOCK_PACKET, 0);
> > >     sendto(fd, NULL, 0, 0, (struct sockaddr *)&addr, sizeof(addr));
> > > 
> > > We have a similar fix for the (AF_PACKET, SOCK_RAW) socket as
> > > commit dc633700f00f ("net/af_packet: check len when min_header_len
> > > equals to 0").
> > > 
> > > Let's add the same test for the SOCK_PACKET socket.
> > > 
> > > [0]:
> > > skb_assert_len
> > > WARNING: CPU: 1 PID: 19945 at include/linux/skbuff.h:2552 skb_assert_len include/linux/skbuff.h:2552 [inline]
> > > WARNING: CPU: 1 PID: 19945 at include/linux/skbuff.h:2552 __dev_queue_xmit+0x1f26/0x31d0 net/core/dev.c:4159
> > > Modules linked in:
> > > CPU: 1 PID: 19945 Comm: syz-executor.0 Not tainted 6.3.0-rc7-02330-gca6270c12e20 #1
> > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> > > RIP: 0010:skb_assert_len include/linux/skbuff.h:2552 [inline]
> > > RIP: 0010:__dev_queue_xmit+0x1f26/0x31d0 net/core/dev.c:4159
> > > Code: 89 de e8 1d a2 85 fd 84 db 75 21 e8 64 a9 85 fd 48 c7 c6 80 2a 1f 86 48 c7 c7 c0 06 1f 86 c6 05 23 cf 27 04 01 e8 fa ee 56 fd <0f> 0b e8 43 a9 85 fd 0f b6 1d 0f cf 27 04 31 ff 89 de e8 e3 a1 85
> > > RSP: 0018:ffff8880217af6e0 EFLAGS: 00010282
> > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffc90001133000
> > > RDX: 0000000000040000 RSI: ffffffff81186922 RDI: 0000000000000001
> > > RBP: ffff8880217af8b0 R08: 0000000000000001 R09: 0000000000000000
> > > R10: 0000000000000001 R11: 0000000000000001 R12: ffff888030045640
> > > R13: ffff8880300456b0 R14: ffff888030045650 R15: ffff888030045718
> > > FS:  00007fc5864da640(0000) GS:ffff88806cd00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000000020005740 CR3: 000000003f856003 CR4: 0000000000770ee0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > PKRU: 55555554
> > > Call Trace:
> > >  <TASK>
> > >  dev_queue_xmit include/linux/netdevice.h:3085 [inline]
> > >  packet_sendmsg_spkt+0xc4b/0x1230 net/packet/af_packet.c:2066
> > >  sock_sendmsg_nosec net/socket.c:724 [inline]
> > >  sock_sendmsg+0x1b4/0x200 net/socket.c:747
> > >  ____sys_sendmsg+0x331/0x970 net/socket.c:2503
> > >  ___sys_sendmsg+0x11d/0x1c0 net/socket.c:2557
> > >  __sys_sendmmsg+0x18c/0x430 net/socket.c:2643
> > >  __do_sys_sendmmsg net/socket.c:2672 [inline]
> > >  __se_sys_sendmmsg net/socket.c:2669 [inline]
> > >  __x64_sys_sendmmsg+0x9c/0x100 net/socket.c:2669
> > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >  do_syscall_64+0x3c/0x90 arch/x86/entry/common.c:80
> > >  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> > > RIP: 0033:0x7fc58791de5d
> > > Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 9f 1b 00 f7 d8 64 89 01 48
> > > RSP: 002b:00007fc5864d9cc8 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> > > RAX: ffffffffffffffda RBX: 00000000004bbf80 RCX: 00007fc58791de5d
> > > RDX: 0000000000000001 RSI: 0000000020005740 RDI: 0000000000000004
> > > RBP: 00000000004bbf80 R08: 0000000000000000 R09: 0000000000000000
> > > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> > > R13: 000000000000000b R14: 00007fc58797e530 R15: 0000000000000000
> > >  </TASK>
> > > ---[ end trace 0000000000000000 ]---
> > > skb len=0 headroom=16 headlen=0 tailroom=304
> > > mac=(16,0) net=(16,-1) trans=-1
> > > shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
> > > csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
> > > hash(0x0 sw=0 l4=0) proto=0x0000 pkttype=0 iif=0
> > > dev name=sit0 feat=0x00000006401d7869
> > > sk family=17 type=10 proto=0
> > > 
> > > Fixes: fd1894224407 ("bpf: Don't redirect packets with invalid pkt_len")
> > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > Reported-by: syzbot <syzkaller@...glegroups.com>
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > 
> > This is a straightforward extension of the referenced commit.
> > 
> > But if the issue is only triggered by BPF shenanigans, as implied
> > by the Fixes and by commit fd1894224407 ("bpf: Don't redirect packets
> > with invalid pkt_len"), which introduces skb_assert_len, can that
> > be catched in BPF redirect, so that we don't have to add assertions
> > and checks elsewhere in the stack?

I remember the v1 of fd1894224407 attempted the way, but BPF folks
wanted to prevent 0-len skb at the source.
https://lore.kernel.org/netdev/f0bf3e9a-15e6-f5c8-1b2a-7866acfcb71b@iogearbox.net/

And skb_assert_len() is suggested here.
https://lore.kernel.org/netdev/7b333bcc-c8ed-f1f8-8331-58cba7897637@iogearbox.net/

> 
> Actually, this commit clearly explains how the 0-len skb is created.
> It does not include BPF. So we should probably remove that Fixes tag.

I followed dc633700f00f which has the same Fixes tag, but I'm fine
with whichever.  I'll post v2 soon.

Thanks!

> 
> Otherwise, looks good to me.
>  
> > > ---
> > >  net/packet/af_packet.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> > > index 6080c0db0814..640d94e34635 100644
> > > --- a/net/packet/af_packet.c
> > > +++ b/net/packet/af_packet.c
> > > @@ -2033,7 +2033,7 @@ static int packet_sendmsg_spkt(struct socket *sock, struct msghdr *msg,
> > >  		goto retry;
> > >  	}
> > >  
> > > -	if (!dev_validate_header(dev, skb->data, len)) {
> > > +	if (!dev_validate_header(dev, skb->data, len) || !skb->len) {
> > >  		err = -EINVAL;
> > >  		goto out_unlock;
> > >  	}
> > > -- 
> > > 2.30.2

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