[PATCH net v3 7/8] xsk: fix xsk_addrs slab leak on multi-buffer error path

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

When xsk_build_skb() / xsk_build_skb_zerocopy() sees the first
continuation descriptor, it promotes destructor_arg from an inlined
address to a freshly allocated xsk_addrs (num_descs = 1). The counter
is bumped to >= 2 only at the very end of a successful build (by calling
xsk_inc_num_desc()).

If the build fails in between (e.g. alloc_page() returns NULL with
-EAGAIN, or the MAX_SKB_FRAGS overflow hits), we jump to free_err, skip
calling xsk_inc_num_desc() to increment num_descs and leave the half-built
skb attached to xs->skb for the app to retry. The skb now has
1) destructor_arg = a real xsk_addrs pointer,
2) num_descs = 1

If the app never retries and just close()s the socket, xsk_release()
calls xsk_drop_skb() -> xsk_consume_skb(), which decides whether to
free xsk_addrs by testing num_descs > 1:

    if (unlikely(num_descs > 1))
        kmem_cache_free(xsk_tx_generic_cache, destructor_arg);

Because num_descs is exactly 1 the branch is skipped and the
xsk_addrs object is leaked to the xsk_tx_generic_cache slab.

Fix it by directly testing if destructor_arg is still addr. Or else it
is modified and used to store the newly allocated memory from
xsk_tx_generic_cache regardless of increment of num_desc, which we
need to handle.

Closes: https://lore.kernel.org/all/20260419045824.D9E5EC2BCAF@smtp.kernel.org/
Fixes: 0ebc27a4c67d ("xsk: avoid data corruption on cq descriptor number")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/xdp/xsk.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 5e6326e076ab..ed96f6ec8ff2 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -605,7 +605,7 @@ static void xsk_cq_submit_addr_locked(struct xsk_buff_pool *pool,
 	spin_lock_irqsave(&pool->cq_prod_lock, flags);
 	idx = xskq_get_prod(pool->cq);
 
-	if (unlikely(num_descs > 1)) {
+	if (unlikely(!xsk_skb_destructor_is_addr(skb))) {
 		xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
 
 		for (i = 0; i < num_descs; i++) {
@@ -660,7 +660,7 @@ static void xsk_consume_skb(struct sk_buff *skb)
 	u32 num_descs = xsk_get_num_desc(skb);
 	struct xsk_addrs *xsk_addr;
 
-	if (unlikely(num_descs > 1)) {
+	if (unlikely(!xsk_skb_destructor_is_addr(skb))) {
 		xsk_addr = (struct xsk_addrs *)skb_shinfo(skb)->destructor_arg;
 		kmem_cache_free(xsk_tx_generic_cache, xsk_addr);
 	}
-- 
2.41.3