[PATCH net v2 1/1] net: smc: fix splice entry lifetime imbalance in smc_rx_splice

[Ren Wei <enjou1224z@gmail.com>]

From: Daming Li <d4n.for.sec@gmail.com>

smc_rx_splice() passes pages to splice_to_pipe() before taking the
references that cover the lifetime of each splice entry. In the
VM-backed RMB path, splice_to_pipe() may drop unqueued entries through
smc_rx_spd_release(), while queued entries are released later via the
pipe buffer callback.

The old post-splice accounting also derives the number of queued VM pages
from an offset mutated while building the descriptor, and a multi-page
splice pairs one sock_hold() with multiple sock_put() calls.

Take the page and socket references for every candidate entry before
splice_to_pipe(), and drop the matching private state, page reference,
and socket reference from smc_rx_spd_release() for entries that never
get queued. This fixes a refcount imbalance that can underflow page
refcounts and trigger a use-after-free.

Fixes: 9014db202cb7 ("smc: add support for splice()")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:GPT-5.4
Co-developed-by: Xiao Liu <lx24@stu.ynu.edu.cn>
Signed-off-by: Xiao Liu <lx24@stu.ynu.edu.cn>
Signed-off-by: Daming Li <d4n.for.sec@gmail.com>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
Reviewed-by: Sidraya Jayagond <sidraya@linux.ibm.com>
---
Changes in v2:
- Keep the offset declaration in its original position to preserve SMC
  reverse-xmas-tree ordering, as requested by Dust Li.
- Add Reviewed-by tags from Dust Li and Sidraya Jayagond.
- Link to v1: https://lore.kernel.org/all/192d1b44ed358ca143f44ef167d14153bccc51e9.1781097957.git.d4n.for.sec@gmail.com/

 net/smc/smc_rx.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/net/smc/smc_rx.c b/net/smc/smc_rx.c
index c1d9b923938d..5c9e4d8b57de 100644
--- a/net/smc/smc_rx.c
+++ b/net/smc/smc_rx.c
@@ -150,7 +150,12 @@ static const struct pipe_buf_operations smc_pipe_ops = {
 static void smc_rx_spd_release(struct splice_pipe_desc *spd,
 			       unsigned int i)
 {
+	struct smc_spd_priv *priv = (struct smc_spd_priv *)spd->partial[i].private;
+	struct sock *sk = &priv->smc->sk;
+
+	kfree(priv);
 	put_page(spd->pages[i]);
+	sock_put(sk);
 }
 
 static int smc_rx_splice(struct pipe_inode_info *pipe, char *src, size_t len,
@@ -209,6 +214,10 @@ static int smc_rx_splice(struct pipe_inode_info *pipe, char *src, size_t len,
 			offset = 0;
 		}
 	}
+	for (i = 0; i < nr_pages; i++) {
+		get_page(pages[i]);
+		sock_hold(&smc->sk);
+	}
 	spd.nr_pages_max = nr_pages;
 	spd.nr_pages = nr_pages;
 	spd.pages = pages;
@@ -217,16 +226,8 @@ static int smc_rx_splice(struct pipe_inode_info *pipe, char *src, size_t len,
 	spd.spd_release = smc_rx_spd_release;
 
 	bytes = splice_to_pipe(pipe, &spd);
-	if (bytes > 0) {
-		sock_hold(&smc->sk);
-		if (!lgr->is_smcd && smc->conn.rmb_desc->is_vm) {
-			for (i = 0; i < PAGE_ALIGN(bytes + offset) / PAGE_SIZE; i++)
-				get_page(pages[i]);
-		} else {
-			get_page(smc->conn.rmb_desc->pages);
-		}
+	if (bytes > 0)
 		atomic_add(bytes, &smc->conn.splice_pending);
-	}
 	kfree(priv);
 	kfree(partial);
 	kfree(pages);

base-commit: 14fa65d10f5696b063a7d8d26e8291ea84a2c6ed
-- 
2.34.1