Re: [PATCH net v2] net: skbuff: propagate shared-frag marker through frag-transfer helpers

[Paolo Abeni <pabeni@redhat.com>]

On 5/13/26 11:07 PM, Hyunwoo Kim wrote:
> Three frag-transfer helpers (__pskb_copy_fclone(), skb_try_coalesce(),
> and skb_shift()) fail to propagate the SKBFL_SHARED_FRAG bit in
> skb_shinfo()->flags when moving frags from source to destination.
> __pskb_copy_fclone() defers the rest of the shinfo metadata to
> skb_copy_header() after copying frag descriptors, but that helper
> only carries over gso_{size,segs,type} and never touches
> skb_shinfo()->flags; skb_try_coalesce() and skb_shift() move frag
> descriptors directly and leave flags untouched.  As a result, 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.
> 
> Fixes: cef401de7be8 ("net: fix possible wrong checksum generation")
> Fixes: f4c50a4034e6 ("xfrm: esp: avoid in-place decrypt on shared skb frags")

WRT the 2nd fixes tag, I *think* f4c50a4034e6 would need
additionally/instead a follow-up similar to the one mentioned by Jakub here:

https://lore.kernel.org/all/20260510084520.476745b5@kernel.org/

/P