configfs: fix a race in configfs_lookup()

commit c42dd069be8dfc9b2239a5c89e73bbd08ab35de0 upstream.
Backporting the patch to stable-v5.10.y to avoid race condition between configfs_dir_lseek and configfs_lookup since they both operate ->s_childre and configfs_lookup
forgets to obtain the lock.
The patch deviates from the original patch because of code change.
The idea is to hold the configfs_dirent_lock when traversing
->s_children, which follows the core idea of the original patch.

---
 fs/configfs/dir.c | 2 ++
 1 file changed, 2 insertions(+)

index 12388ed4faa5..0b7e9ab517d5 100644
@@ -479,6 +479,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
 	if (!configfs_dirent_is_ready(parent_sd))
 		goto out;
 
+	spin_lock(&configfs_dirent_lock);
 	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
 		if (sd->s_type & CONFIGFS_NOT_PINNED) {
 			const unsigned char * name = configfs_get_name(sd);

@@ -491,6 +492,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
 			break;
 		}
 	}
+	spin_unlock(&configfs_dirent_lock);
 
 	if (!found) {
 		/*

2.34.1

You lost all the original signed-off-by lines of the original, AND you lost the authorship of the original commit. And you didn't cc: anyone involved in the original patch, to get their review, or objection to it being backported.

Take a look at many of the backports that happen on the stable list for examples of how to do this properly.

Here are 2 examples from this weekend alone that are good examples of how to do this properly:

	https://lore.kernel.org/r/[email protected]
	https://lore.kernel.org/r/[email protected]

Also, how did you test this change? is this something that you have actually hit in real life?

thanks,

greg k-h

Sorry for the rookie mistakes. I drafted another version and it is attached to the email. Can you please check whether it is OK?

Yes. I encountered this when testing the latest v5.10.y branch. A minimal proof-of-concept code looks like this:
~~~
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int main(void)

{
	int fd = open("/sys/kernel/config", 0);

	if(!fork()) {
		while(1) lseek(fd, SEEK_CUR, 1);
	}
	while(1) unlinkat(fd, "file", 0);

	return 0;
}

~~~

From 366d165e876a14a5b25ad741fdcd8658aa7e690d Mon Sep 17 00:00:00 2001
From: Kyle Zeng <[email protected]>
Date: Fri, 1 Sep 2023 17:41:14 -0700
Subject: [PATCH v5.10.y] configfs: fix a race in configfs_lookup()

commit c42dd069be8dfc9b2239a5c89e73bbd08ab35de0 upstream.

configfs: fix a race in configfs_lookup()
When configfs_lookup() is executing list_for_each_entry(), it is possible that configfs_dir_lseek() is calling list_del(). Some unfortunate interleavings of them can cause a kernel NULL pointer dereference error

Thread 1 Thread 2
//configfs_dir_lseek() //configfs_lookup()

list_del(&cursor->s_sibling);
                         list_for_each_entry(sd, ...)

Fix this by grabbing configfs_dirent_lock in configfs_lookup() while iterating ->s_children.

---
Please apply this patch to v5.10.y. This patch adapts the original patch to v5.10.y considering codebase change.
The idea is to hold the configfs_dirent_lock when traversing ->s_children, which follows the core idea of the original patch.

This patch has been tested against the v5.10.y stable tree.

 fs/configfs/dir.c | 2 ++
 1 file changed, 2 insertions(+)

index 12388ed4faa5..0b7e9ab517d5 100644
@@ -479,6 +479,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
 	if (!configfs_dirent_is_ready(parent_sd))
 		goto out;
 
+	spin_lock(&configfs_dirent_lock);
 	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
 		if (sd->s_type & CONFIGFS_NOT_PINNED) {
 			const unsigned char * name = configfs_get_name(sd);

@@ -491,6 +492,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
 			break;
 		}
 	}
+	spin_unlock(&configfs_dirent_lock);
 
 	if (!found) {
 		/*

2.34.1

Good enough, thanks! Now queued up.

greg k-h