net: skbuff: preserve shared-frag marker during coalescing

commit f84eca5 upstream.

skb_try_coalesce() can attach paged frags from @from to @to.  If @from
has SKBFL_SHARED_FRAG set, the resulting @to skb can contain the same
externally-owned or page-cache-backed frags, but the shared-frag marker
is currently lost.

That breaks the invariant relied on by later in-place writers.  In
particular, ESP input checks skb_has_shared_frag() before deciding
whether an uncloned nonlinear skb can skip skb_cow_data().  If TCP
receive coalescing has moved shared frags into an unmarked skb, ESP can
see skb_has_shared_frag() as false and decrypt in place over page-cache
backed frags.

Propagate SKBFL_SHARED_FRAG when skb_try_coalesce() transfers paged
frags.  The tailroom copy path does not need the marker because it copies
bytes into @to's linear data rather than transferring frag descriptors.

Fixes: cef401d ("net: fix possible wrong checksum generation")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Signed-off-by: William Bowling <vakzz@zellic.io>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Tested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260513041635.1289541-1-vakzz@zellic.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: wbowling

net/core/skbuff.c

@@ -5512,6 +5512,8 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 	       from_shinfo->frags,
 	       from_shinfo->nr_frags * sizeof(skb_frag_t));
 	to_shinfo->nr_frags += from_shinfo->nr_frags;
+	if (from_shinfo->nr_frags)
+		to_shinfo->flags |= from_shinfo->flags & SKBFL_SHARED_FRAG;
 
 	if (!skb_cloned(from))
 		from_shinfo->nr_frags = 0;