Subject: Re: [PATCH net] net: skbuff: propagate shared-frag marker through pskb_copy()

From: Hyunwoo Kim <[email-masked]>
Date: Thu, 14 May 2026 02:16:31 +0900

On Wed, May 13, 2026 at 06:21:45PM +0200, Ben Hutchings wrote:
> On Wed, 2026-05-13 at 20:25 +0900, Hyunwoo Kim wrote:
> > __pskb_copy_fclone() shallow-copies the source's frag descriptors and
> > bumps each page's refcount via skb_frag_ref(), then defers the rest
> > of the shinfo metadata to skb_copy_header().  That helper only carries
> > over gso_{size,segs,type} and never touches skb_shinfo()->flags, so
> > the destination skb keeps a reference to the same externally-owned or
> > page-cache-backed pages while reporting skb_has_shared_frag() as
> > false.
> >
> > The mismatch is harmful in any in-place writer that uses
> > skb_has_shared_frag() to decide whether shared pages must be detoured
> > through skb_cow_data().  ESP input is one such writer (esp4.c,
> > esp6.c), and a single nft 'dup to <local>' rule -- or any other
> > nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d
> > skb in esp_input() with the marker stripped, letting an unprivileged
> > user write into the page cache of a root-owned read-only file via
> > authencesn-ESN stray writes.
> > 
> > Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors
> > were actually moved from the source.  skb_copy() and skb_copy_expand()
> > share skb_copy_header() too but linearize all paged data into freshly
> > allocated head storage and emerge with nr_frags == 0, so
> > skb_has_shared_frag() returns false on its own; they need no change.
> 
> What about skb_shift()?  It seems like that should also propagate this
> flag.  But I could be missing some reason why it's not necessary.

Yes, since skb_shift() is also a function that moves frag descriptors, 
I think SHARED_FRAG should be propagated as well. The actual trigger 
conditions are tricky (not deterministic) due to TCP write-queue skb 
merging, but I believe the fix is the right thing to do. 

I'm planning to submit a v2 patch. What do you think?

Best regards,
Hyunwoo Kim