[PATCH AUTOSEL 6.18] xsk: tighten UMEM headroom validation to account for tailroom and min frame

[Sasha Levin]

From: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>

[ Upstream commit a315e022a72d95ef5f1d4e58e903cb492b0ad931 ]

The current headroom validation in xdp_umem_reg() could leave us with
insufficient space dedicated to even receive minimum-sized ethernet
frame. Furthermore if multi-buffer would come to play then
skb_shared_info stored at the end of XSK frame would be corrupted.

HW typically works with 128-aligned sizes so let us provide this value
as bare minimum.

Multi-buffer setting is known later in the configuration process so
besides accounting for 128 bytes, let us also take care of tailroom space
upfront.

Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxx>
Acked-by: Stanislav Fomichev <sdf@xxxxxxxxxxx>
Fixes: 99e3a236dd43 ("xsk: Add missing check on user supplied headroom size")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@xxxxxxxxx>
Link: https://patch.msgid.link/20260402154958.562179-2-maciej.fijalkowski@xxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---

LLM Generated explanations, may be completely bogus:

Error: Failed to generate final synthesis

 net/xdp/xdp_umem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
index 9f76ca591d54f..9ec7bd948acc7 100644
--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -202,7 +202,8 @@ static int xdp_umem_reg(struct xdp_umem *umem, struct xdp_umem_reg *mr)
 	if (!unaligned_chunks && chunks_rem)
 		return -EINVAL;
 
-	if (headroom >= chunk_size - XDP_PACKET_HEADROOM)
+	if (headroom > chunk_size - XDP_PACKET_HEADROOM -
+		       SKB_DATA_ALIGN(sizeof(struct skb_shared_info)) - 128)
 		return -EINVAL;
 
 	if (mr->flags & XDP_UMEM_TX_METADATA_LEN) {
-- 
2.53.0