[PATCH 6.12.y] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

[Suraj Jitindar Singh <surajjs@amazon.com>]

From: Jason Xing <kernelxing@tencent.com>

commit 22ba97ea9cc1f63a0d0244fae38057ed452b6ac7 upstream.

The TX metadata area resides in the UMEM buffer which is memory-mapped
and concurrently writable by userspace. In xsk_skb_metadata(),
csum_start and csum_offset are read from shared memory for bounds
validation, then read again for skb assignment. A malicious userspace
application can race to overwrite these values between the two reads,
bypassing the bounds check and causing out-of-bounds memory access
during checksum computation in the transmit path.

Fix this by reading csum_start and csum_offset into local variables
once, then using the local copies for both validation and assignment.

Note that other metadata fields (flags, launch_time) and the cached
csum fields may be mutually inconsistent due to concurrent userspace
writes, but this is benign: the only security-critical invariant is
that each field's validated value is the same one used, which local
caching guarantees.

Closes: https://lore.kernel.org/all/20260503200927.73EA1C2BCB4@smtp.kernel.org/
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support")
Link: https://patch.msgid.link/20260530042630.80626-1-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ 6.12: Apply fix in xsk_build_skb() since missing patch
	30c3055f9c0d ("xsk: wrap generic metadata handling onto separate
	function") added in v6.18 which moves handling to separate
	function xsk_skb_metadata(). ]
Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>
---
 net/xdp/xsk.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index da7e11e3bfad..76675ce70f03 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -665,6 +665,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 	struct xsk_tx_metadata *meta = NULL;
 	struct net_device *dev = xs->dev;
 	struct sk_buff *skb = xs->skb;
+	u16 csum_start, csum_offset;
 	bool first_frag = false;
 	int err;
 
@@ -733,15 +734,17 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			}
 
 			if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) {
-				if (unlikely(meta->request.csum_start +
-					     meta->request.csum_offset +
+				csum_start = READ_ONCE(meta->request.csum_start);
+				csum_offset = READ_ONCE(meta->request.csum_offset);
+
+				if (unlikely(csum_start + csum_offset +
 					     sizeof(__sum16) > len)) {
 					err = -EINVAL;
 					goto free_err;
 				}
 
-				skb->csum_start = hr + meta->request.csum_start;
-				skb->csum_offset = meta->request.csum_offset;
+				skb->csum_start = hr + csum_start;
+				skb->csum_offset = csum_offset;
 				skb->ip_summed = CHECKSUM_PARTIAL;
 
 				if (unlikely(xs->pool->tx_sw_csum)) {
-- 
2.47.3