6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Daehyeon Ko <4ncienth@gmail.com> commit 8b1f0af1fd5fd113432233ed8a3862ee7c30a84f upstream. After copying a readable prefix, receive_fallback_to_copy() asks tcp_zerocopy_set_hint_for_skb() where page mapping can resume. If the next skb is unreadable, find_next_mappable_frag() passes its net_iov fragment to can_map_frag(). skb_frag_page() returns NULL for a net_iov, but can_map_frag() dereferences it in PageCompound(). A v7.2 KASAN run on a connected TCP socket with 64 readable bytes followed by a 4096-byte NET_IOV_DMABUF fragment reported: BUG: KASAN: null-ptr-deref in can_map_frag tcp_zerocopy_receive -> can_map_frag Kernel panic - not syncing: KASAN: panic_on_warn set The diagnostic inserted the net_iov directly because the test host has no devmem-capable NIC. Hardware end-to-end reachability remains untested and requires CONFIG_NET_DEVMEM plus a supported DMA-buf-bound RX queue. Reject all net_iov fragments before skb_frag_page(). This covers both DMABUF and IOURING net_iov types while leaving page-backed checks unchanged. With the guard, the same queue copied the readable prefix, returned a 4096-byte skip hint, and completed without a fault. Fixes: 9f6b619edf2e ("net: support non paged skb frags") Cc: stable@vger.kernel.org Signed-off-by: Daehyeon Ko <4ncienth@gmail.com> Reviewed-by: Mina Almasry Link: https://patch.msgid.link/20260930102524.1659847-1-4ncienth@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Greg Kroah-Hartman --- net/ipv4/tcp.c | 2 ++ 1 file changed, 2 insertions(+) --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -1883,6 +1883,8 @@ static bool can_map_frag(const skb_frag_ if (skb_frag_size(frag) != PAGE_SIZE || skb_frag_off(frag)) return false; + if (skb_frag_is_net_iov(frag)) + return false; page = skb_frag_page(frag);