Re: [PATCH net] net: skbuff: reject invalid pull bounds

On Sun, Aug 23, 2026 at 4:26 PM Shihuang Liu <[email protected]> wrote:
>
> skb_maybe_pull_tail() subtracts skb_headlen(skb) from the unsigned max
> argument and passes the result to __pskb_pull_tail() as a signed int.  The
> function does not ensure that max is at least skb_headlen(skb).
>
> This can happen while parsing IPv6 extension headers when an skb already
> has a linear area larger than MAX_IPV6_HDR_LEN.  Once the parser needs data
> beyond the linear area, max - skb_headlen(skb) wraps and is converted to a
> negative delta.  __pskb_pull_tail() then passes that negative length to
> skb_copy_bits(), where it can become a very large copy length.
>
> Reject this inconsistent bound before doing the subtraction.  In this case
> pulling up to max cannot satisfy len because the initial check established
> that skb_headlen(skb) is smaller than len.
>
> Fixes: 1431fb31ecba ("xen-netback: fix fragment detection in checksum setup")
> Cc: [email protected]
> Assisted-by: LLM

> Signed-off-by: Shihuang Liu <[email protected]>
> ---
>  net/core/skbuff.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index c82a147..33022f7 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5838,6 +5838,9 @@ static int skb_maybe_pull_tail(struct sk_buff *skb, unsigned int len,
>         if (max > skb->len)
>                 max = skb->len;
>
> +       if (max < skb_headlen(skb))
> +               return -EPROTO;
> +

pw-bot: rejected

Please ask your LLM not to slow down the fast path, and find a better
fix in skb_checksum_setup_ipv6().

Note how skb_checksum_setup_ip() is fine.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d4382b68d56e0b3b247868b4d29ab383cfcf7440..f77d7480594ca6a1dcce2fea737e421d5ea2d38d
100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5974,7 +5974,8 @@ static int skb_checksum_setup_ipv6(struct
sk_buff *skb, bool recalculate)
                        err = skb_maybe_pull_tail(skb,
                                                  off +
                                                  sizeof(struct ipv6_opt_hdr),
-                                                 MAX_IPV6_HDR_LEN);
+                                                 off +
+                                                 sizeof(struct ipv6_opt_hdr));
                        if (err < 0)
                                goto out;

@@ -5989,7 +5990,8 @@ static int skb_checksum_setup_ipv6(struct
sk_buff *skb, bool recalculate)
                        err = skb_maybe_pull_tail(skb,
                                                  off +
                                                  sizeof(struct ip_auth_hdr),
-                                                 MAX_IPV6_HDR_LEN);
+                                                 off +
+                                                 sizeof(struct ip_auth_hdr));
                        if (err < 0)
                                goto out;

@@ -6004,7 +6006,8 @@ static int skb_checksum_setup_ipv6(struct
sk_buff *skb, bool recalculate)
                        err = skb_maybe_pull_tail(skb,
                                                  off +
                                                  sizeof(struct frag_hdr),
-                                                 MAX_IPV6_HDR_LEN);
+                                                 off +
+                                                 sizeof(struct frag_hdr));
                        if (err < 0)
                                goto out;