linux-kernel - [PATCH] nbd: fix uaf in nbd_genl

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Message-Id: <20250612132405.364904-1-zhengqixing@huaweicloud.com>
Date: Thu, 12 Jun 2025 21:24:05 +0800
From: Zheng Qixing <zhengqixing@...weicloud.com>
To: josef@...icpanda.com,
	axboe@...nel.dk,
	xiubli@...hat.com,
	prasanna.kalever@...hat.com,
	ming.lei@...hat.com
Cc: linux-block@...r.kernel.org,
	nbd@...er.debian.org,
	linux-kernel@...r.kernel.org,
	yukuai3@...wei.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	zhengqixing@...wei.com
Subject: [PATCH] nbd: fix uaf in nbd_genl_connect() error path

From: Zheng Qixing <zhengqixing@...wei.com>

There is a use-after-free issue in nbd:

block nbd6: Receive control failed (result -104)
block nbd6: shutting down sockets
==================================================================
BUG: KASAN: slab-use-after-free in recv_work+0x694/0xa80 drivers/block/nbd.c:1022
Write of size 4 at addr ffff8880295de478 by task kworker/u33:0/67

CPU: 2 UID: 0 PID: 67 Comm: kworker/u33:0 Not tainted 6.15.0-rc5-syzkaller-00123-g2c89c1b655c0 #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Workqueue: nbd6-recv recv_work
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:408 [inline]
 print_report+0xc3/0x670 mm/kasan/report.c:521
 kasan_report+0xe0/0x110 mm/kasan/report.c:634
 check_region_inline mm/kasan/generic.c:183 [inline]
 kasan_check_range+0xef/0x1a0 mm/kasan/generic.c:189
 instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
 atomic_dec include/linux/atomic/atomic-instrumented.h:592 [inline]
 recv_work+0x694/0xa80 drivers/block/nbd.c:1022
 process_one_work+0x9cc/0x1b70 kernel/workqueue.c:3238
 process_scheduled_works kernel/workqueue.c:3319 [inline]
 worker_thread+0x6c8/0xf10 kernel/workqueue.c:3400
 kthread+0x3c2/0x780 kernel/kthread.c:464
 ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

nbd_genl_connect() does not properly stop the device on certain
error paths after nbd_start_device() has been called. This causes
the error path to put nbd->config while recv_work continue to use
the config after putting it, leading to use-after-free in recv_work.

This patch moves nbd_start_device() after the backend file creation.

Reported-by: syzbot+48240bab47e705c53126@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/68227a04.050a0220.f2294.00b5.GAE@google.com/T/
Fixes: 6497ef8df568 ("nbd: provide a way for userspace processes to identify device backends")
Signed-off-by: Zheng Qixing <zhengqixing@...wei.com>
---
 drivers/block/nbd.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 7bdc7eb808ea..2592bd19ebc1 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2198,9 +2198,7 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 				goto out;
 		}
 	}
-	ret = nbd_start_device(nbd);
-	if (ret)
-		goto out;
+
 	if (info->attrs[NBD_ATTR_BACKEND_IDENTIFIER]) {
 		nbd->backend = nla_strdup(info->attrs[NBD_ATTR_BACKEND_IDENTIFIER],
 					  GFP_KERNEL);
@@ -2216,6 +2214,8 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 		goto out;
 	}
 	set_bit(NBD_RT_HAS_BACKEND_FILE, &config->runtime_flags);
+
+	ret = nbd_start_device(nbd);
 out:
 	mutex_unlock(&nbd->config_lock);
 	if (!ret) {
-- 
2.39.2