xsk: Validate xsk_tx_metadata flags

Accept only the flags that the kernel knows about to make
sure we can extend this field in the future. Note that only
in XDP_COPY mode we propagate the error signal back to the user
(via sendmsg). For zerocopy mode we silently skip the metadata
for the descriptors that have wrong flags (since we process
the descriptors deep in the driver).
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20231127190319.1190813-8-sdf@google.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/net/xdp_sock_drv.h

@@ -165,12 +165,28 @@ static inline void *xsk_buff_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
 	return xp_raw_get_data(pool, addr);
 }
 
+#define XDP_TXMD_FLAGS_VALID ( \
+		XDP_TXMD_FLAGS_TIMESTAMP | \
+		XDP_TXMD_FLAGS_CHECKSUM | \
+	0)
+
+static inline bool xsk_buff_valid_tx_metadata(struct xsk_tx_metadata *meta)
+{
+	return !(meta->flags & ~XDP_TXMD_FLAGS_VALID);
+}
+
 static inline struct xsk_tx_metadata *xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
 {
+	struct xsk_tx_metadata *meta;
+
 	if (!pool->tx_metadata_len)
 		return NULL;
 
-	return xp_raw_get_data(pool, addr) - pool->tx_metadata_len;
+	meta = xp_raw_get_data(pool, addr) - pool->tx_metadata_len;
+	if (unlikely(!xsk_buff_valid_tx_metadata(meta)))
+		return NULL; /* no way to signal the error to the user */
+
+	return meta;
 }
 
 static inline void xsk_buff_dma_sync_for_cpu(struct xdp_buff *xdp, struct xsk_buff_pool *pool)
@@ -332,6 +348,11 @@ static inline void *xsk_buff_raw_get_data(struct xsk_buff_pool *pool, u64 addr)
 	return NULL;
 }
 
+static inline bool xsk_buff_valid_tx_metadata(struct xsk_tx_metadata *meta)
+{
+	return false;
+}
+
 static inline struct xsk_tx_metadata *xsk_buff_get_metadata(struct xsk_buff_pool *pool, u64 addr)
 {
 	return NULL;

net/xdp/xsk.c

@@ -728,6 +728,10 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			}
 
 			meta = buffer - xs->pool->tx_metadata_len;
+			if (unlikely(!xsk_buff_valid_tx_metadata(meta))) {
+				err = -EINVAL;
+				goto free_err;
+			}
 
 			if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) {
 				if (unlikely(meta->request.csum_start +