[PATCH v3] scsi: target: fix recursive locking in __configfs_open_file()

[Prithvi Tambewagh <activprithvi@gmail.com>]

In flush_write_buffer, &p->frag_sem is acquired and then the loaded store
function is called, which, here, is target_core_item_dbroot_store().
This function called filp_open(), following which these functions were
called (in reverse order), according to the call trace:

down_read
__configfs_open_file
do_dentry_open
vfs_open
do_open
path_openat
do_filp_open
file_open_name
filp_open
target_core_item_dbroot_store
flush_write_buffer
configfs_write_iter

target_core_item_dbroot_store() tries to validate the new file path by
trying to open the file path provided to it; however, in this case,
the bug report shows:

db_root: not a directory: /sys/kernel/config/target/dbroot

indicating that the same configfs file was tried to be opened, on which
it is currently working on. Thus, it is trying to acquire frag_sem
semaphore of the same file of which it already holds the semaphore obtained
in flush_write_buffer(), leading to acquiring the semaphore in a nested
manner and a possibility of recursive locking.

Fix this by modifying target_core_item_dbroot_store() to use kern_path()
instead of filp_open() to avoid opening the file using filesystem-specific
function __configfs_open_file(), and further modifying it to make this
fix compatible.

Reported-by: syzbot+f6e8174215573a84b797@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f6e8174215573a84b797
Tested-by: syzbot+f6e8174215573a84b797@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Prithvi Tambewagh <activprithvi@gmail.com>
Reviewed-by: Dmitry Bogdanov <d.bogdanov@yadro.com>
---
Changes since v2:
 - Add Reviewed-by tag received from Dmitry Bogdanov, which was accidentally
   left to be added in v2 patch.

v2 link: https://lore.kernel.org/linux-scsi/20260122154051.64132-1-activprithvi@gmail.com/T/#u
Reference for Reviewed-by Tag: https://lore.kernel.org/all/20260108191523.303114-1-activprithvi@gmail.com/T/#mb22d0fc06e747e2b2df8320a15afd2a0670fd0e7


Changes since v1:
 - Update commit message to reflect the fact that same file, which code was 
   currently operating on, was tried to be opened again, leading to 
   acquiring the same semaphore in nested manner & possibility of recursive
   locking.

v1 link: https://lore.kernel.org/all/20260108191523.303114-1-activprithvi@gmail.com/T/

 drivers/target/target_core_configfs.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/drivers/target/target_core_configfs.c b/drivers/target/target_core_configfs.c
index b19acd662726..f29052e6a87d 100644
--- a/drivers/target/target_core_configfs.c
+++ b/drivers/target/target_core_configfs.c
@@ -108,8 +108,8 @@ static ssize_t target_core_item_dbroot_store(struct config_item *item,
 					const char *page, size_t count)
 {
 	ssize_t read_bytes;
-	struct file *fp;
 	ssize_t r = -EINVAL;
+	struct path path = {};
 
 	mutex_lock(&target_devices_lock);
 	if (target_devices) {
@@ -131,17 +131,18 @@ static ssize_t target_core_item_dbroot_store(struct config_item *item,
 		db_root_stage[read_bytes - 1] = '\0';
 
 	/* validate new db root before accepting it */
-	fp = filp_open(db_root_stage, O_RDONLY, 0);
-	if (IS_ERR(fp)) {
+	r = kern_path(db_root_stage, LOOKUP_FOLLOW, &path);
+	if (r) {
 		pr_err("db_root: cannot open: %s\n", db_root_stage);
 		goto unlock;
 	}
-	if (!S_ISDIR(file_inode(fp)->i_mode)) {
-		filp_close(fp, NULL);
+	if (!d_is_dir(path.dentry)) {
+		path_put(&path);
 		pr_err("db_root: not a directory: %s\n", db_root_stage);
+		r = -ENOTDIR;
 		goto unlock;
 	}
-	filp_close(fp, NULL);
+	path_put(&path);
 
 	strscpy(db_root, db_root_stage);
 	pr_debug("Target_Core_ConfigFS: db_root set to %s\n", db_root);

base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
-- 
2.34.1