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

From: David Laight <[email-masked]>
Date: Wed, 27 May 2026 17:13:10 +0100

On Wed, 27 May 2026 10:56:57 -0400
Jamal Hadi Salim <[email-masked]> wrote:

>  &&
> 
> On Tue, May 26, 2026 at 3:22 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
> >
> > 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;
> > }
> >  
> 
> Yes, that improves readability. If i understood the discussion between
> you and David L. something like this one liner would be reasonable?
> 
> if (offset > (int)skb->len || offset < -(int)skb_headroom(skb))

I doubt it will be measurable whatever you pick.

Thinks, can 'we' get away with the much more readable:
	offset + (int)skb_headroom(skb) < 0
No need to worry about the '+' wrapping, the first test will fail.
Large -ve offset are going to stay negative.
So it does look all right.

> 
> 
> > > @@ -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?
> >  
> 
> I focused on the bigger solution and worried more about timeliness to
> get this patch in - but it seems the distros had already picked up the
> first posted patch, so hakuna matata (Still, I should have caught
> these unlikelies ;->). Yes on the second one you pointed out.
> Will remove them.
> 
> > > +                                             &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).  
> 
> hrm. So:
> if ((write_offset < 0 && ((skb_cow(skb, -write_offset)) &&
> (write_offset + (int)sizeof(*ptr) > 0) &&
> (skb_ensure_writable(skb,min(skb->len,write_offset + sizeof(*ptr)))) {
>             goto bad;
> else {
>   ...
> }
> 
> Not sure which is more readable.

I think he meant just the last two 'if'.

-- David

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