Re: [PATCH 4.14 16/68] net: ensure mac header is set in virtio_net_hdr_to_skb()

[Balazs Nemeth]

On Thu, 2021-04-15 at 16:46 +0200, Greg Kroah-Hartman wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> commit 61431a5907fc36d0738e9a547c7e1556349a03e9 upstream.
> 
> Commit 924a9bc362a5 ("net: check if protocol extracted by
> virtio_net_hdr_set_proto is correct")
> added a call to dev_parse_header_protocol() but mac_header is not yet
> set.
> 
> This means that eth_hdr() reads complete garbage, and syzbot
> complained about it [1]
> 
> This patch resets mac_header earlier, to get more coverage about this
> change.
> 
> Audit of virtio_net_hdr_to_skb() callers shows that this change
> should be safe.
> 
> [1]
> 
> BUG: KASAN: use-after-free in eth_header_parse_protocol+0xdc/0xe0
> net/ethernet/eth.c:282
> Read of size 2 at addr ffff888017a6200b by task syz-executor313/8409
> 
> CPU: 1 PID: 8409 Comm: syz-executor313 Not tainted 5.12.0-rc2-
> syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine,
> BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x141/0x1d7 lib/dump_stack.c:120
>  print_address_description.constprop.0.cold+0x5b/0x2f8
> mm/kasan/report.c:232
>  __kasan_report mm/kasan/report.c:399 [inline]
>  kasan_report.cold+0x7c/0xd8 mm/kasan/report.c:416
>  eth_header_parse_protocol+0xdc/0xe0 net/ethernet/eth.c:282
>  dev_parse_header_protocol include/linux/netdevice.h:3177 [inline]
>  virtio_net_hdr_to_skb.constprop.0+0x99d/0xcd0
> include/linux/virtio_net.h:83
>  packet_snd net/packet/af_packet.c:2994 [inline]
>  packet_sendmsg+0x2325/0x52b0 net/packet/af_packet.c:3031
>  sock_sendmsg_nosec net/socket.c:654 [inline]
>  sock_sendmsg+0xcf/0x120 net/socket.c:674
>  sock_no_sendpage+0xf3/0x130 net/core/sock.c:2860
>  kernel_sendpage.part.0+0x1ab/0x350 net/socket.c:3631
>  kernel_sendpage net/socket.c:3628 [inline]
>  sock_sendpage+0xe5/0x140 net/socket.c:947
>  pipe_to_sendpage+0x2ad/0x380 fs/splice.c:364
>  splice_from_pipe_feed fs/splice.c:418 [inline]
>  __splice_from_pipe+0x43e/0x8a0 fs/splice.c:562
>  splice_from_pipe fs/splice.c:597 [inline]
>  generic_splice_sendpage+0xd4/0x140 fs/splice.c:746
>  do_splice_from fs/splice.c:767 [inline]
>  do_splice+0xb7e/0x1940 fs/splice.c:1079
>  __do_splice+0x134/0x250 fs/splice.c:1144
>  __do_sys_splice fs/splice.c:1350 [inline]
>  __se_sys_splice fs/splice.c:1332 [inline]
>  __x64_sys_splice+0x198/0x250 fs/splice.c:1332
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
> 
> Fixes: 924a9bc362a5 ("net: check if protocol extracted by
> virtio_net_hdr_set_proto is correct")
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Balazs Nemeth <bnemeth@xxxxxxxxxx>
> Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/virtio_net.h |    2 ++
>  1 file changed, 2 insertions(+)
> 
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -62,6 +62,8 @@ static inline int virtio_net_hdr_to_skb(
>                         return -EINVAL;
>         }
>  
> +       skb_reset_mac_header(skb);
> +
>         if (hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) {
>                 u16 start = __virtio16_to_cpu(little_endian, hdr-
> >csum_start);
>                 u16 off = __virtio16_to_cpu(little_endian, hdr-
> >csum_offset);
> 
> 

Hi,

Since the call to dev_parse_header_protocol is only made for gso
packets where skb->protocol is not set, we could move
skb_reset_mac_header down closer to that call. Is there another reason
to reset mac_header earlier (and affect handling of other packets as
well)? In any case, thanks for spotting this!

Regards,
Balazs