net: gro: don't merge zcopy skbs

[ Upstream commit 4db79a322db8c97f7b73b8a347395ef4d685eb40 ]

skb_gro_receive() can currently copy frags between the source and GRO
skb, without checking the zerocopy status, and in particular the
SKBFL_MANAGED_FRAG_REFS flag.

When SKBFL_MANAGED_FRAG_REFS is set, the skb doesn't hold a reference
on the pages in shinfo->frags. Appending those frags to another skb's
frags without fixing up the page refcount can lead to UAF.

When either the last skb in the GRO chain (the one we would append
frags to) or the source skb is zerocopy, don't merge the skbs.

BUG=b/522218060
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-46323 in the Linux kernel.

cos-patch:security-high
Fixes: 753f1ca4e1e5 ("net: introduce managed frags infrastructure")
Reported-by: Huzaifa Sidhpurwala <huzaifas@redhat.com>
Change-Id: I2ca35a83b532dadd9c839f34ce8226cfb9334680
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/c3b7f906bbfcbdfd7b4fa9d6c18a438870df85be.1779307748.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/160204
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Daniel Velasquez <rdvelasquez@google.com>