[PATCH net] xdp: fix zero-copy frame layout

xdp_convert_zc_to_xdp_frame() clones an XSK packet into an order-0 page
and advertises PAGE_SIZE as its frame size.  It allows the copied frame
to occupy the page tail needed by skb_shared_info and records zero
headroom even when metadata separates the frame header from packet data.
An AF_XDP zero-copy packet redirected through cpumap can therefore make
the skb overlap skb_shared_info or place it beyond the allocated page.

Limit the copied layout to SKB_WITH_OVERHEAD(PAGE_SIZE) and include the
metadata length in frame headroom.  Redirect callers already handle a
NULL conversion result.

BUG: KASAN: slab-out-of-bounds in skb_gro_receive
Write of size 4 at addr ffff88800cf37004 by task cpumap/1/map:1/146
Call Trace:
 skb_gro_receive (net/core/gro.c:174)
 udp_gro_receive (net/ipv4/udp_offload.c:812)
 inet_gro_receive (net/ipv4/af_inet.c:1539)
 dev_gro_receive (net/core/gro.c:515)
 gro_receive_skb (net/core/gro.c:633)
 cpu_map_kthread_run (kernel/bpf/cpumap.c:395)
 kthread (kernel/kthread.c:436)
 ret_from_fork (arch/x86/kernel/process.c:164)
 ret_from_fork_asm (arch/x86/entry/entry_64.S:255)
Kernel panic - not syncing: KASAN: panic_on_warn set ...

Fixes: b0d1beeff2a9 ("xdp: implement convert_to_xdp_frame for MEM_TYPE_ZERO_COPY")
Cc: [email protected]
Reported-by: Xiang Mei <[email protected]>
Assisted-by: Codex:gpt-5.6
Signed-off-by: Weiming Shi <[email protected]>
---
 net/core/xdp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/xdp.c b/net/core/xdp.c
index 9890a30584ba..4bb97790b189 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -585,7 +585,7 @@ struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp)
 		   xdp->data - xdp->data_meta;
 	totsize = xdp->data_end - xdp->data + metasize;
 
-	if (sizeof(*xdpf) + totsize > PAGE_SIZE)
+	if (sizeof(*xdpf) + totsize > SKB_WITH_OVERHEAD(PAGE_SIZE))
 		return NULL;
 
 	page = dev_alloc_page();
@@ -602,7 +602,7 @@ struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp)
 
 	xdpf->data = addr + metasize;
 	xdpf->len = totsize - metasize;
-	xdpf->headroom = 0;
+	xdpf->headroom = metasize;
 	xdpf->metasize = metasize;
 	xdpf->frame_sz = PAGE_SIZE;
 	xdpf->mem_type = MEM_TYPE_PAGE_ORDER0;
-- 
2.55.0