Subject: Re: [PATCH v2] net: missing check virtio

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Jun 2024 03:51:08 -0400

On Thu, Jun 13, 2024 at 12:54:48PM +0300, Denis Arefev wrote:
> Two missing check in virtio_net_hdr_to_skb() allowed syzbot
> to crash kernels again
> 
> 1. After the skb_segment function the buffer may become non-linear
> (nr_frags != 0), but since the SKBTX_SHARED_FRAG flag is not set anywhere
> the __skb_linearize function will not be executed, then the buffer will
> remain non-linear. Then the condition (offset >= skb_headlen(skb))
> becomes true, which causes WARN_ON_ONCE in skb_checksum_help.
> 
> 2. The struct sk_buff and struct virtio_net_hdr members must be
> mathematically related.
> (gso_size) must be greater than (needed) otherwise WARN_ON_ONCE.
> (remainder) must be greater than (needed) otherwise WARN_ON_ONCE.
> (remainder) may be 0 if division is without remainder.
> 
> offset+2 (4191) > skb_headlen() (1116)
> WARNING: CPU: 1 PID: 5084 at net/core/dev.c:3303 skb_checksum_help+0x5e2/0x740 net/core/dev.c:3303
> Modules linked in:
> CPU: 1 PID: 5084 Comm: syz-executor336 Not tainted 6.7.0-rc3-syzkaller-00014-gdf60cee26a2e #0
> Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 11/10/2023
> RIP: 0010:skb_checksum_help+0x5e2/0x740 net/core/dev.c:3303
> Code: 89 e8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 52 01 00 00 44 89 e2 2b 53 74 4c 89 ee 48 c7 c7 40 57 e9 8b e8 af 8f dd f8 90 <0f> 0b 90 90 e9 87 fe ff ff e8 40 0f 6e f9 e9 4b fa ff ff 48 89 ef
> RSP: 0018:ffffc90003a9f338 EFLAGS: 00010286
> RAX: 0000000000000000 RBX: ffff888025125780 RCX: ffffffff814db209
> RDX: ffff888015393b80 RSI: ffffffff814db216 RDI: 0000000000000001
> RBP: ffff8880251257f4 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000001 R12: 000000000000045c
> R13: 000000000000105f R14: ffff8880251257f0 R15: 000000000000105d
> FS:  0000555555c24380(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000002000f000 CR3: 0000000023151000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  ip_do_fragment+0xa1b/0x18b0 net/ipv4/ip_output.c:777
>  ip_fragment.constprop.0+0x161/0x230 net/ipv4/ip_output.c:584
>  ip_finish_output_gso net/ipv4/ip_output.c:286 [inline]
>  __ip_finish_output net/ipv4/ip_output.c:308 [inline]
>  __ip_finish_output+0x49c/0x650 net/ipv4/ip_output.c:295
>  ip_finish_output+0x31/0x310 net/ipv4/ip_output.c:323
>  NF_HOOK_COND include/linux/netfilter.h:303 [inline]
>  ip_output+0x13b/0x2a0 net/ipv4/ip_output.c:433
>  dst_output include/net/dst.h:451 [inline]
>  ip_local_out+0xaf/0x1a0 net/ipv4/ip_output.c:129
>  iptunnel_xmit+0x5b4/0x9b0 net/ipv4/ip_tunnel_core.c:82
>  ipip6_tunnel_xmit net/ipv6/sit.c:1034 [inline]
>  sit_tunnel_xmit+0xed2/0x28f0 net/ipv6/sit.c:1076
>  __netdev_start_xmit include/linux/netdevice.h:4940 [inline]
>  netdev_start_xmit include/linux/netdevice.h:4954 [inline]
>  xmit_one net/core/dev.c:3545 [inline]
>  dev_hard_start_xmit+0x13d/0x6d0 net/core/dev.c:3561
>  __dev_queue_xmit+0x7c1/0x3d60 net/core/dev.c:4346
>  dev_queue_xmit include/linux/netdevice.h:3134 [inline]
>  packet_xmit+0x257/0x380 net/packet/af_packet.c:276
>  packet_snd net/packet/af_packet.c:3087 [inline]
>  packet_sendmsg+0x24ca/0x5240 net/packet/af_packet.c:3119
>  sock_sendmsg_nosec net/socket.c:730 [inline]
>  __sock_sendmsg+0xd5/0x180 net/socket.c:745
>  __sys_sendto+0x255/0x340 net/socket.c:2190
>  __do_sys_sendto net/socket.c:2202 [inline]
>  __se_sys_sendto net/socket.c:2198 [inline]
>  __x64_sys_sendto+0xe0/0x1b0 net/socket.c:2198
>  do_syscall_x64 arch/x86/entry/common.c:51 [inline]
>  do_syscall_64+0x40/0x110 arch/x86/entry/common.c:82
>  entry_SYSCALL_64_after_hwframe+0x63/0x6b
> 
> Found by Linux Verification Center (linuxtesting.org) with Syzkaller
> 
> Signed-off-by: Denis Arefev <[email-masked]>
> ---
>  V1 -> V2: incorrect type in argument 2
>  include/linux/virtio_net.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 4dfa9b69ca8d..d1d7825318c3 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -56,6 +56,7 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  	unsigned int thlen = 0;
>  	unsigned int p_off = 0;
>  	unsigned int ip_proto;
> +	u64 ret, remainder, gso_size;
>  
>  	if (hdr->gso_type != VIRTIO_NET_HDR_GSO_NONE) {
>  		switch (hdr->gso_type & ~VIRTIO_NET_HDR_GSO_ECN) {
> @@ -98,6 +99,16 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  		u32 off = __virtio16_to_cpu(little_endian, hdr->csum_offset);
>  		u32 needed = start + max_t(u32, thlen, off + sizeof(__sum16));
>  
> +		if (hdr->gso_size) {
> +			gso_size = __virtio16_to_cpu(little_endian, hdr->gso_size);
> +			ret = div64_u64_rem(skb->len, gso_size, &remainder);
> +			if (!(ret && (hdr->gso_size > needed) &&
> +						((remainder > needed) || (remainder == 0)))) {
> +				return -EINVAL;
> +			}

Could you please add some comments explaining the logic here?

And do we really need u64 math? All the values here are 32 bit ATM ...

> +			skb_shinfo(skb)->tx_flags |= SKBFL_SHARED_FRAG;
> +		}
> +
>  		if (!pskb_may_pull(skb, needed))
>  			return -EINVAL;
>  
> -- 
> 2.25.1