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

From: David Laight <[email-masked]>
Date: Tue, 26 May 2026 10:53:06 +0100

On Mon, 18 May 2026 20:39:50 -0700
Rajat Gupta <[email-masked]> wrote:

> 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: Rajat Gupta <[email-masked]>
> 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: Han Guidong <[email-masked]>
> Acked-by: Jamal Hadi Salim <[email-masked]>
> Signed-off-by: Rajat Gupta <[email-masked]>
> ---
>  net/sched/act_pedit.c | 54 ++++++++++++++++++++++++++++++++-----------
>  1 file changed, 41 insertions(+), 13 deletions(-)
> 
> diff --git a/net/sched/act_pedit.c b/net/sched/act_pedit.c
> index bc20f08a2..79921b8d8 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>
> @@ -323,8 +324,10 @@ 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))
> -		return false;
> +	if (offset < 0) {
> +		if (offset == INT_MIN || -offset > skb_headroom(skb))
> +			return false;
> +	}

Can't you negate skb_headroom() instead - that cannot be INT_MIN.
So:
	if (offset < 0 && offset < -skb_headroom(skb))

>  
>  	return true;
>  }
> @@ -393,17 +396,21 @@ 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;
> +	/*
> +	 * If the skb has shared frags the user is likely using zero-copy
> +	 * (e.g. sendfile).  Those page frags may point to page-cache pages;
> +	 * writing into them would silently corrupt the page cache.
> +	 * Linearize so pedit operates on a private copy.
> +	 * TL;DR: if you want zero-copy, don't use pedit.
> +	 */
> +	if (skb_has_shared_frag(skb)) {
> +		if (__skb_linearize(skb))
> +			goto bad;
> +	}

Should there be a way of 'unsharing' frags by just copying the frags
rather than doing a full linearize?
That would be much less likely to fail for big skb.

-- David

>  
>  	tcf_lastuse_update(&p->tcf_tm);
>  	tcf_action_update_bstats(&p->common, skb);
> @@ -412,6 +419,7 @@ 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;
> @@ -451,12 +459,32 @@ 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,
> +						&write_offset))) {
> +			pr_info_ratelimited("tc action pedit offset overflow\n");
> +			goto bad;
> +		}
> +
> +		if (!offset_valid(skb, write_offset)) {
> +			pr_info_ratelimited("tc action pedit offset %d out of bounds\n",
> +					    write_offset);
>  			goto bad;
>  		}
>  
> -		ptr = skb_header_pointer(skb, hoffset + offset,
> +		if (write_offset < 0) {
> +			if (skb_cow(skb, -write_offset))
> +				goto bad;
> +		} else {
> +			if (unlikely(check_add_overflow(write_offset,
> +							(int)sizeof(hdata),
> +							&write_len)))
> +				goto bad;
> +			if (skb_ensure_writable(skb, min_t(int, skb->len,
> +							   write_len)))
> +				goto bad;
> +		}
> +
> +		ptr = skb_header_pointer(skb, write_offset,
>  					 sizeof(hdata), &hdata);
>  		if (!ptr)
>  			goto bad;
> @@ -475,7 +503,7 @@ 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);
> +			skb_store_bits(skb, write_offset, ptr, sizeof(hdata));
>  	}
>  
>  	goto done;