netdev - Re: [PATCH v3 1/2] Fixes: 924a9bc362a5 ("net: check if protocol extracted by virtio_net_hdr_set

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Date:   Mon, 1 Aug 2022 10:13:35 +0200
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Cezar Bulinaru <cbulinaru@...il.com>
Cc:     davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v3 1/2] Fixes: 924a9bc362a5 ("net: check if protocol
 extracted by virtio_net_hdr_set_proto is correct")

On Mon, Aug 1, 2022 at 6:57 AM Cezar Bulinaru <cbulinaru@...il.com> wrote:
>
> Fixes a NULL pointer derefence bug triggered from tap driver.
> When tap_get_user calls virtio_net_hdr_to_skb the skb->dev is null
> (in tap.c skb->dev is set after the call to virtio_net_hdr_to_skb)
> virtio_net_hdr_to_skb calls dev_parse_header_protocol which
> needs skb->dev field to be valid.
>
> The line that trigers the bug is in dev_parse_header_protocol
> (dev is at offset 0x10 from skb and is stored in RAX register)
>   if (!dev->header_ops || !dev->header_ops->parse_protocol)
>   22e1:   mov    0x10(%rbx),%rax
>   22e5:   mov    0x230(%rax),%rax
>
> Setting skb->dev before the call in tap.c fixes the issue.
>
> BUG: kernel NULL pointer dereference, address: 0000000000000230
> RIP: 0010:virtio_net_hdr_to_skb.constprop.0+0x335/0x410 [tap]
> Code: c0 0f 85 b7 fd ff ff eb d4 41 39 c6 77 cf 29 c6 48 89 df 44 01 f6 e8 7a 79 83 c1 48 85 c0 0f 85 d9 fd ff ff eb b7 48 8b 43 10 <48> 8b 80 30 02 00 00 48 85 c0 74 55 48 8b 40 28 48 85 c0 74 4c 48
> RSP: 0018:ffffc90005c27c38 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888298f25300 RCX: 0000000000000010
> RDX: 0000000000000005 RSI: ffffc90005c27cb6 RDI: ffff888298f25300
> RBP: ffffc90005c27c80 R08: 00000000ffffffea R09: 00000000000007e8
> R10: ffff88858ec77458 R11: 0000000000000000 R12: 0000000000000001
> R13: 0000000000000014 R14: ffffc90005c27e08 R15: ffffc90005c27cb6
> FS:  0000000000000000(0000) GS:ffff88858ec40000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000230 CR3: 0000000281408006 CR4: 00000000003706e0
> Call Trace:
>  tap_get_user+0x3f1/0x540 [tap]
>  tap_sendmsg+0x56/0x362 [tap]
>  ? get_tx_bufs+0xc2/0x1e0 [vhost_net]
>  handle_tx_copy+0x114/0x670 [vhost_net]
>  handle_tx+0xb0/0xe0 [vhost_net]
>  handle_tx_kick+0x15/0x20 [vhost_net]
>  vhost_worker+0x7b/0xc0 [vhost]
>  ? vhost_vring_call_reset+0x40/0x40 [vhost]
>  kthread+0xfa/0x120
>  ? kthread_complete_and_exit+0x20/0x20
>  ret_from_fork+0x1f/0x30
>
> Signed-off-by: Cezar Bulinaru <cbulinaru@...il.com>

There is something wrong with the subject line.

The Fixes tag is still missing too.

Small comments inside, but overall the code looks good to me as is, too.

>
> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> index c3d42062559d..557236d51d01 100644
> --- a/drivers/net/tap.c
> +++ b/drivers/net/tap.c
> @@ -716,10 +716,20 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>         skb_reset_mac_header(skb);
>         skb->protocol = eth_hdr(skb)->h_proto;
>
> +       rcu_read_lock();
> +       tap = rcu_dereference(q->tap);
> +       if (tap) {
> +               skb->dev = tap->dev;
> +       } else {
> +               kfree_skb(skb);
> +               goto post_send;
> +       }
> +

I would just

  if (!tap) {
          rcu_read_unlock();
          kfree_skb(skb);
          return total_len;
  }

I agree to not change the code beyond the strict bug fix, but slight
aside that it seems weird that this code returns success on this
failure, and that it could use kfree_skb_reason
(SKB_DROP_REASON_DEV_READY?).

>         if (vnet_hdr_len) {
>                 err = virtio_net_hdr_to_skb(skb, &vnet_hdr,
>                                             tap_is_little_endian(q));
>                 if (err) {
> +                       rcu_read_unlock();
>                         drop_reason = SKB_DROP_REASON_DEV_HDR;
>                         goto err_kfree;
>                 }
> @@ -732,8 +742,6 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>             __vlan_get_protocol(skb, skb->protocol, &depth) != 0)
>                 skb_set_network_header(skb, depth);
>
> -       rcu_read_lock();
> -       tap = rcu_dereference(q->tap);
>         /* copy skb_ubuf_info for callback when skb has no error */
>         if (zerocopy) {
>                 skb_zcopy_init(skb, msg_control);
> @@ -742,12 +750,9 @@ static ssize_t tap_get_user(struct tap_queue *q, void *msg_control,
>                 uarg->callback(NULL, uarg, false);
>         }
>
> -       if (tap) {
> -               skb->dev = tap->dev;
> -               dev_queue_xmit(skb);
> -       } else {
> -               kfree_skb(skb);
> -       }
> +       dev_queue_xmit(skb);
> +
> +post_send:
>         rcu_read_unlock();
>
>         return total_len;
> --
> 2.34.1
>