Re: [PATCH net] net: Add check for csum_start in skb_partial_csum_set()

[Eric Dumazet]

On Mon, Apr 10, 2023 at 4:22 AM Lu Wei <luwei32@xxxxxxxxxx> wrote:
>
> If an AF_PACKET socket is used to send packets through a L3 mode ipvlan
> and a vnet header is set via setsockopt() with the option name of
> PACKET_VNET_HDR, the value of offset will be nagetive in function
> skb_checksum_help() and trigger the following warning:
>
> WARNING: CPU: 3 PID: 2023 at net/core/dev.c:3262
> skb_checksum_help+0x2dc/0x390
> ......
> Call Trace:
>  <TASK>
>  ip_do_fragment+0x63d/0xd00
>  ip_fragment.constprop.0+0xd2/0x150
>  __ip_finish_output+0x154/0x1e0
>  ip_finish_output+0x36/0x1b0
>  ip_output+0x134/0x240
>  ip_local_out+0xba/0xe0
>  ipvlan_process_v4_outbound+0x26d/0x2b0
>  ipvlan_xmit_mode_l3+0x44b/0x480
>  ipvlan_queue_xmit+0xd6/0x1d0
>  ipvlan_start_xmit+0x32/0xa0
>  dev_hard_start_xmit+0xdf/0x3f0
>  packet_snd+0xa7d/0x1130
>  packet_sendmsg+0x7b/0xa0
>  sock_sendmsg+0x14f/0x160
>  __sys_sendto+0x209/0x2e0
>  __x64_sys_sendto+0x7d/0x90
>
> The root cause is:
> 1. skb->csum_start is set in packet_snd() according vnet_hdr:
>    skb->csum_start = skb_headroom(skb) + (u32)start;
>
>    'start' is the offset from skb->data, and mac header has been
>    set at this moment.
>
> 2. when this skb arrives ipvlan_process_outbound(), the mac header
>    is unset and skb_pull is called to expand the skb headroom.
>
> 3. In function skb_checksum_help(), the variable offset is calculated
>    as:
>       offset = skb->csum_start - skb_headroom(skb);
>
>    since skb headroom is expanded in step2, offset is nagetive, and it
>    is converted to an unsigned integer when compared with skb_headlen
>    and trigger the warning.

Not sure why it is negative ? This seems like the real problem...

csum_start is relative to skb->head, regardless of pull operations.

whatever set csum_start to a too small value should be tracked and fixed.

>
> In fact the data to be checksummed should not contain the mac header
> since the mac header is stripped after a packet leaves L2 layer.
> This patch fixes this by adding a check for csum_start to make it
> start after the mac header.
>
> Fixes: 52b5d6f5dcf0 ("net: make skb_partial_csum_set() more robust against overflows")
> Signed-off-by: Lu Wei <luwei32@xxxxxxxxxx>
> ---
>  net/core/skbuff.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1a31815104d6..5e24096076fa 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5232,9 +5232,11 @@ bool skb_partial_csum_set(struct sk_buff *skb, u16 start, u16 off)
>         u32 csum_end = (u32)start + (u32)off + sizeof(__sum16);
>         u32 csum_start = skb_headroom(skb) + (u32)start;
>
> -       if (unlikely(csum_start > U16_MAX || csum_end > skb_headlen(skb))) {
> -               net_warn_ratelimited("bad partial csum: csum=%u/%u headroom=%u headlen=%u\n",
> -                                    start, off, skb_headroom(skb), skb_headlen(skb));
> +       if (unlikely(csum_start > U16_MAX || csum_end > skb_headlen(skb) ||
> +                    csum_start < skb->network_header)) {
> +               net_warn_ratelimited("bad partial csum: csum=%u/%u headroom=%u headlen=%u network_header=%u\n",
> +                                    start, off, skb_headroom(skb),
> +                                    skb_headlen(skb), skb->network_header);
>

I do not understand this patch. You are working around the real bug, right ?

Otherwise we would not have a net_warn_ratelimited() ?

csum_start should actually be at the transport header, so not
considering network header
 length seems to call for another bug report when syzbot gets smarter ?