[PATCH RFC] netdevsim: fix deadlock in nsim_bus_dev_max_vfs_write()

["syzbot" <syzbot@kernel.org>]

There is an ABBA deadlock between the devlink instance lock and the debugfs
active file reference mechanism.

When a device is being removed (e.g., via nsim_drv_remove()), the driver
acquires the devlink instance lock and then calls
debugfs_remove_recursive(). This function blocks waiting for all active
file operations on the debugfs files to complete.

Concurrently, if a user writes to the "max_vfs" debugfs file, the VFS layer
acquires an active reference to the file. The write handler,
nsim_bus_dev_max_vfs_write(), then attempts to acquire the devlink instance
lock, which is already held by the removal task.

This creates a circular dependency resulting in a deadlock:

INFO: task blocked for more than 143 seconds.
Call Trace:
 wait_for_completion+0x2ca/0x5e0 kernel/sched/completion.c:153
 __debugfs_file_removed fs/debugfs/inode.c:751 [inline]
 remove_one+0x2df/0x3b0 fs/debugfs/inode.c:758
 __simple_recursive_removal+0x215/0x520 fs/libfs.c:623
 debugfs_remove+0x5b/0x70 fs/debugfs/inode.c:781
 nsim_dev_debugfs_exit drivers/net/netdevsim/dev.c:372 [inline]
 nsim_drv_remove+0xc0/0x170 drivers/net/netdevsim/dev.c:1803

INFO: task blocked for more than 143 seconds.
Call Trace:
 __mutex_lock+0x7bf/0x1550 kernel/locking/mutex.c:821
 nsim_bus_dev_max_vfs_write+0x229/0x3d0 drivers/net/netdevsim/dev.c:276
 full_proxy_write+0x127/0x1f0 fs/debugfs/file.c:388
 vfs_write+0x296/0xba0 fs/read_write.c:685

To fix this, use devl_trylock() in nsim_bus_dev_max_vfs_write() instead of
devl_lock(). If the lock cannot be acquired, return -EBUSY. This aborts the
write operation, releases the debugfs active file reference, and allows the
pending debugfs_remove_recursive() to proceed.

Fixes: 012ec02ae441 ("netdevsim: convert driver to use unlocked devlink API during init/fini")
Assisted-by: Gemini:gemini-3.1-pro-preview best-expensive syzbot
Reported-by: syzbot+3147c5de186107ffc7a1@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3147c5de186107ffc7a1
Link: https://syzkaller.appspot.com/ai_job?id=e3832a24-8c9f-416a-8161-a50e277bc628
To: "Andrew Lunn" <andrew+netdev@lunn.ch>
To: "David S. Miller" <davem@davemloft.net>
To: "Eric Dumazet" <edumazet@google.com>
To: "Jakub Kicinski" <kuba@kernel.org>
To: <netdev@vger.kernel.org>
To: "Paolo Abeni" <pabeni@redhat.com>
Cc: <linux-kernel@vger.kernel.org>

---
diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
index aed9ad5f1..421cd7327 100644
--- a/drivers/net/netdevsim/dev.c
+++ b/drivers/net/netdevsim/dev.c
@@ -273,7 +273,11 @@ static ssize_t nsim_bus_dev_max_vfs_write(struct file *file,
 		return -ENOMEM;
 
 	nsim_dev = file->private_data;
-	devl_lock(priv_to_devlink(nsim_dev));
+	if (!devl_trylock(priv_to_devlink(nsim_dev))) {
+		ret = -EBUSY;
+		goto out;
+	}
+
 	/* Reject if VFs are configured */
 	if (nsim_dev_get_vfs(nsim_dev)) {
 		ret = -EBUSY;
@@ -285,6 +289,7 @@ static ssize_t nsim_bus_dev_max_vfs_write(struct file *file,
 	}
 	devl_unlock(priv_to_devlink(nsim_dev));
 
+out:
 	kfree(vfconfigs);
 	return ret;
 }


base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
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