net: gro: propagate SKBFL_SHARED_FRAG in skb_gro_receive()

skb_gro_receive() moves frag descriptors from the incoming skb to the
GRO accumulator through two frag-transfer paths (the direct frag-move
loop and the head_frag + memcpy path) without propagating the
SKBFL_SHARED_FRAG flag from the incoming skb's shinfo->flags. As a
result, the accumulator ends up holding references to externally owned
or page-cache-backed pages while reporting skb_has_shared_frag() as
false.

This is the same bug class as CVE-2026-46300 (d8cfbcd, "net:
skbuff: propagate shared-frag marker through frag-transfer helpers"),
which fixed the identical omission in __pskb_copy_fclone(),
skb_try_coalesce(), and skb_shift(). skb_gro_receive() was missed in
that fix since it lives in net/core/gro.c rather than net/core/skbuff.c.

The impact is observable through ESP-over-UDP with UDP GRO: splice()
attaches page-cache pages to a UDP skb, setting SKBFL_SHARED_FRAG via
ip_append_page(). When two such datagrams are GRO-merged via
skb_gro_receive(), the flag is dropped. After udp_rcv_segment()
re-segments the merged GSO skb, the fresh segments carry the
page-cache frags without the shared-frag marker. esp_input() then sees
\!skb_cloned() && \!skb_has_shared_frag() and takes the skip_cow fast
path, decrypting in place over the page-cache pages. Because AES-GCM
CTR decryption runs before the authentication tag is verified, the
page cache is corrupted even though the tag check subsequently fails.

Fix it by propagating SKBFL_SHARED_FRAG from the incoming skb to the
accumulator in both frag-transfer paths, matching what the skbuff.c
helpers already do. The third path (frag_list merge at the "merge:"
label) chains the entire incoming skb onto the accumulator's frag_list
without moving individual frag descriptors, so each sub-skb retains
its own flags and no propagation is needed there.

Fixes: cef401d ("net: fix possible wrong checksum generation")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Cc: stable@vger.kernel.org
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Sultan Alsawaf <sultan@ciq.com>

Author: shreeya-patel98

net/core/skbuff.c

@@ -4519,6 +4519,8 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 		skb_frag_off_add(frag, offset);
 		skb_frag_size_sub(frag, offset);
 
+		pinfo->flags |= skbinfo->flags & SKBFL_SHARED_FRAG;
+
 		/* all fragments truesize : remove (head size + sk_buff) */
 		new_truesize = SKB_TRUESIZE(skb_end_offset(skb));
 		delta_truesize = skb->truesize - new_truesize;
@@ -4552,6 +4554,8 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 		memcpy(frag + 1, skbinfo->frags, sizeof(*frag) * skbinfo->nr_frags);
 		/* We dont need to clear skbinfo->nr_frags here */
 
+		pinfo->flags |= skbinfo->flags & SKBFL_SHARED_FRAG;
+
 		new_truesize = SKB_DATA_ALIGN(sizeof(struct sk_buff));
 		delta_truesize = skb->truesize - new_truesize;
 		skb->truesize = new_truesize;