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

[Eric Dumazet]

On Fri, Apr 16, 2021 at 10:49 AM Balazs Nemeth <bnemeth@xxxxxxxxxx> wrote:
>
> 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!
>

The answer to your question was in the changelog

"This patch resets mac_header earlier, to get more coverage about this change."

We want to detect if such a reset is going to hurt in general, not
only for GSO packets.