Re: [PATCH net v3 1/1] net/sched: fix pedit partial COW leading to page cache corruption

[Han Guidong]

On Thu, May 28, 2026 at 2:17 AM Jamal Hadi Salim <jhs@xxxxxxxxxxxx> wrote:
>
> From: Rajat Gupta <rajat.gupta@xxxxxxxxxxxxxxxx>
>
> tcf_pedit_act() computes the COW range for skb_ensure_writable()
> once before the key loop using tcfp_off_max_hint, but the hint does
> not account for the runtime header offset added by typed keys. This
> can leave part of the write region un-COW'd.
>
> Fix by moving skb_ensure_writable() inside the per-key loop where
> the actual write offset is known, and add overflow checking on the
> offset arithmetic. For negative offsets (e.g. Ethernet header edits
> at ingress), use skb_cow() to COW the headroom instead. Guard
> offset_valid() against INT_MIN, where negation is undefined.
>
> Reported-by: Yiming Qian <yimingqian591@xxxxxxxxx>
> Reported-by: Keenan Dong <keenanat2000@xxxxxxxxx>
> Reported-by: Han Guidong <2045gemini@xxxxxxxxx>
> Reported-by: Zhang Cen <rollkingzzc@xxxxxxxxx>
> Reviewed-by: Han Guidong <2045gemini@xxxxxxxxx>
> Tested-by: Han Guidong <2045gemini@xxxxxxxxx>
> Reviewed-by: Davide Caratti <dcaratti@xxxxxxxxxx>
> Tested-by: Davide Caratti <dcaratti@xxxxxxxxxx>
> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> Tested-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> Reviewed-by: Victor Nogueira <victor@xxxxxxxxxxxx>
> Tested-by: Victor Nogueira <victor@xxxxxxxxxxxx>
> Acked-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>
> Signed-off-by: Rajat Gupta <rajat.gupta@xxxxxxxxxxxxxxxx>
> ---
> Changes v2->v3:
> 1) Fixup commit log not to talk about linearize
> 2) Add more optimal boundary checks (Toke & David L.)
> 3) Fixup if statement for readability (Toke)

Retested this with various debug options enabled, and everything still
looks good here.

Thanks.

> ---
>  net/sched/act_pedit.c | 45 +++++++++++++++++++++++++++----------------
>  1 file changed, 28 insertions(+), 17 deletions(-)
>
> diff --git a/net/sched/act_pedit.c b/net/sched/act_pedit.c
> index bc20f08a2789..e01865af8ca0 100644
> --- a/net/sched/act_pedit.c
> +++ b/net/sched/act_pedit.c
> @@ -16,6 +16,7 @@
>  #include <linux/ip.h>
>  #include <linux/ipv6.h>
>  #include <linux/slab.h>
> +#include <linux/overflow.h>
>  #include <net/ipv6.h>
>  #include <net/netlink.h>
>  #include <net/pkt_sched.h>
> @@ -323,7 +324,7 @@ static bool offset_valid(struct sk_buff *skb, int offset)
>         if (offset > 0 && offset > skb->len)
>                 return false;
>
> -       if  (offset < 0 && -offset > skb_headroom(skb))
> +       if (offset < 0 && offset < -(int)skb_headroom(skb))
>                 return false;
>
>         return true;
> @@ -393,18 +394,10 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
>         struct tcf_pedit_key_ex *tkey_ex;
>         struct tcf_pedit_parms *parms;
>         struct tc_pedit_key *tkey;
> -       u32 max_offset;
>         int i;
>
>         parms = rcu_dereference_bh(p->parms);
>
> -       max_offset = (skb_transport_header_was_set(skb) ?
> -                     skb_transport_offset(skb) :
> -                     skb_network_offset(skb)) +
> -                    parms->tcfp_off_max_hint;
> -       if (skb_ensure_writable(skb, min(skb->len, max_offset)))
> -               goto done;
> -
>         tcf_lastuse_update(&p->tcf_tm);
>         tcf_action_update_bstats(&p->common, skb);
>
> @@ -412,9 +405,10 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
>         tkey_ex = parms->tcfp_keys_ex;
>
>         for (i = parms->tcfp_nkeys; i > 0; i--, tkey++) {
> +               int write_offset, write_len;
>                 int offset = tkey->off;
>                 int hoffset = 0;
> -               u32 *ptr, hdata;
> +               u32 *ptr;
>                 u32 val;
>                 int rc;
>
> @@ -451,15 +445,34 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
>                         }
>                 }
>
> -               if (!offset_valid(skb, hoffset + offset)) {
> -                       pr_info_ratelimited("tc action pedit offset %d out of bounds\n", hoffset + offset);
> +               if (check_add_overflow(hoffset, offset, &write_offset)) {
> +                       pr_info_ratelimited("tc action pedit offset overflow\n");
>                         goto bad;
>                 }
>
> -               ptr = skb_header_pointer(skb, hoffset + offset,
> -                                        sizeof(hdata), &hdata);
> -               if (!ptr)
> +               if (!offset_valid(skb, write_offset)) {
> +                       pr_info_ratelimited("tc action pedit offset %d out of bounds\n",
> +                                           write_offset);
>                         goto bad;
> +               }
> +
> +               if (write_offset < 0) {
> +                       if (skb_cow(skb, -write_offset))
> +                               goto bad;
> +                       if (write_offset + (int)sizeof(*ptr) > 0 &&
> +                           skb_ensure_writable(skb, min(skb->len,
> +                                                        write_offset + sizeof(*ptr))))
> +                               goto bad;
> +               } else {
> +                       if (check_add_overflow(write_offset, (int)sizeof(*ptr),
> +                                              &write_len))
> +                               goto bad;
> +                       if (skb_ensure_writable(skb, min_t(int, skb->len,
> +                                                          write_len)))
> +                               goto bad;
> +               }
> +
> +               ptr = (u32 *)(skb->data + write_offset);
>                 /* just do it, baby */
>                 switch (cmd) {
>                 case TCA_PEDIT_KEY_EX_CMD_SET:
> @@ -474,8 +487,6 @@ TC_INDIRECT_SCOPE int tcf_pedit_act(struct sk_buff *skb,
>                 }
>
>                 *ptr = ((*ptr & tkey->mask) ^ val);
> -               if (ptr == &hdata)
> -                       skb_store_bits(skb, hoffset + offset, ptr, 4);
>         }
>
>         goto done;
> --
> 2.34.1
>