Subject: Re: [PATCH net v2 1/1] net/sched: fix pedit partial COW leading to page cache corruption

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 26 May 2026 21:22:52 +0200

Jamal Hadi Salim <[email-masked]> writes:

> From: Rajat Gupta <[email-masked]>
>
> 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.
>
> Additionally, linearize skbs with shared frags upfront to prevent
> silent data corruption when pedit operates on zero-copy pages
> (e.g. from sendfile).
>
> Fixes: 8b796475fd78 ("net/sched: act_pedit: really ensure the skb is writable")
> Reported-by: Yiming Qian <[email-masked]>
> Reported-by: Keenan Dong <[email-masked]>
> Reported-by: Han Guidong <[email-masked]>
> Reported-by: Zhang Cen <[email-masked]>
> Tested-by: Victor Nogueira <[email-masked]>
> Acked-by: Jamal Hadi Salim <[email-masked]>
> Signed-off-by: Rajat Gupta <[email-masked]>

Re-ran the tests, and everything looks good, so:

Tested-by: Toke Høiland-Jørgensen <[email-masked]>

Also looked at the code, and I have a few nits below, but I'm really
nitpicking here, so whether you end up fixing those or not:

Reviewed-by: Toke Høiland-Jørgensen <[email-masked]>

[...]

> @@ -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;

This change makes it really obvious that this is really just:

	if (offset < -(int)skb_headroom(skb))
  		return false;

so, well, that would be clearer, IMO.

But then I guess the same could be said of the positive case, so:

static bool offset_valid(struct sk_buff *skb, int offset)
{
	if (offset > skb->len || 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 +445,38 @@ 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 (unlikely(check_add_overflow(hoffset, offset,

It's a bit weird that this has the unlikely(), but the offset_valid()
check doesn't?

> +						&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) {
> +				if (skb_ensure_writable(skb,
> +							min(skb->len,
> +							    write_offset + sizeof(*ptr))))

Combining these with && instead of the double indentation would be more
readable IMO (shorter lines, aligning the 'goto bad' labels).

> +					goto bad;
> +			}
> +		} else {
> +			if (unlikely(check_add_overflow(write_offset,
> +							(int)sizeof(*ptr),
> +							&write_len)))

Same comment wrt unlikely()