[PATCH net v2] xsk: reject undersized tx_metadata at runtime validation

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

There is no metadata length check before accessing struct xsk_tx_metadata
fields in both xdp_umem_reg() (registration) and
xsk_buff_valid_tx_metadata() (runtime).  Thus, an invalid small
tx_metadata_len (e.g. 8 bytes) passes registration and later causes an
out-of-bounds read when xsk_skb_metadata() accesses fields beyond the
declared metadata region.

KASAN reports this as:

  BUG: KASAN: vmalloc-out-of-bounds in xsk_skb_metadata+0x4b2/0x500
  Read of size 8 at addr ffffc90000f11000 by task exploit/148

  xsk_skb_metadata (net/xdp/xsk.c:837)
  xsk_build_skb (net/xdp/xsk.c)
  __xsk_generic_xmit (net/xdp/xsk.c)
  xsk_sendmsg (net/xdp/xsk.c)

Fixing at registration time would break backward compatibility with older
userspace applications, so apply a runtime length check in
xsk_buff_valid_tx_metadata() that validates tx_metadata_len against the
fields required by the enabled flags.

Fixes: 341ac980eab9 ("xsk: Support tx_metadata_len")
Reported-by: AutonomousCodeSecurity@microsoft.com
Link: https://lore.kernel.org/all/20260720155210.34229-1-blbllhy@gmail.com/
Suggested-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
V2:
 - Change to runtime per-flag length check as suggested by Stanislav.

 include/net/xdp_sock_drv.h | 22 ++++++++++++++++++----
 net/xdp/xsk.c              |  2 +-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/include/net/xdp_sock_drv.h b/include/net/xdp_sock_drv.h
index 46797645a0c2..5c54c7462c45 100644
--- a/include/net/xdp_sock_drv.h
+++ b/include/net/xdp_sock_drv.h
@@ -260,9 +260,22 @@ xsk_buff_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr)
 	0)
 
 static inline bool
-xsk_buff_valid_tx_metadata(const struct xsk_tx_metadata *meta)
+xsk_buff_valid_tx_metadata(const struct xsk_buff_pool *pool,
+			   const struct xsk_tx_metadata *meta)
 {
-	return !(meta->flags & ~XDP_TXMD_FLAGS_VALID);
+	/* covers flags, XDP_TXMD_FLAGS_CHECKSUM & XDP_TXMD_FLAGS_TIMESTAMP */
+	if (unlikely(pool->tx_metadata_len < 16))
+		return false;
+
+	if (unlikely(meta->flags & ~XDP_TXMD_FLAGS_VALID))
+		return false;
+
+	if (meta->flags & XDP_TXMD_FLAGS_LAUNCH_TIME)
+		if (unlikely(pool->tx_metadata_len <
+			     offsetofend(struct xsk_tx_metadata, request.launch_time)))
+			return false;
+
+	return true;
 }
 
 static inline struct xsk_tx_metadata *
@@ -274,7 +287,7 @@ __xsk_buff_get_metadata(const struct xsk_buff_pool *pool, void *data)
 		return NULL;
 
 	meta = data - pool->tx_metadata_len;
-	if (unlikely(!xsk_buff_valid_tx_metadata(meta)))
+	if (unlikely(!xsk_buff_valid_tx_metadata(pool, meta)))
 		return NULL; /* no way to signal the error to the user */
 
 	return meta;
@@ -469,7 +482,8 @@ xsk_buff_raw_get_ctx(const struct xsk_buff_pool *pool, u64 addr)
 	return (struct xdp_desc_ctx){ };
 }
 
-static inline bool xsk_buff_valid_tx_metadata(struct xsk_tx_metadata *meta)
+static inline bool xsk_buff_valid_tx_metadata(const struct xsk_buff_pool *pool,
+					      struct xsk_tx_metadata *meta)
 {
 	return false;
 }
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index b970f30ea9b9..75b2c97e41e4 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -809,7 +809,7 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
 		return -EINVAL;
 
 	meta = buffer - pool->tx_metadata_len;
-	if (unlikely(!xsk_buff_valid_tx_metadata(meta)))
+	if (unlikely(!xsk_buff_valid_tx_metadata(pool, meta)))
 		return -EINVAL;
 
 	if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) {
-- 
2.53.0