linux-kernel - Re: [PATCH] scsi: target: Fix recursive locking in __configfs_open

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Message-ID: <20260122142933.662srnqeqmx7eqk3@inspiron>
Date: Thu, 22 Jan 2026 19:59:33 +0530
From: Prithvi <activprithvi@...il.com>
To: Dmitry Bogdanov <d.bogdanov@...ro.com>
Cc: martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	hch@....de, jlbec@...lplan.org, linux-fsdevel@...r.kernel.org,
	linux-kernel-mentees@...ts.linux.dev, skhan@...uxfoundation.org,
	david.hunter.linux@...il.com, khalid@...nel.org,
	syzbot+f6e8174215573a84b797@...kaller.appspotmail.com,
	stable@...r.kernel.org
Subject: Re: [PATCH] scsi: target: Fix recursive locking in
 __configfs_open_file()

On Thu, Jan 22, 2026 at 12:56:34PM +0300, Dmitry Bogdanov wrote:
> On Thu, Jan 15, 2026 at 08:50:12AM +0530, Prithvi wrote:
> > 
> > On Fri, Jan 09, 2026 at 12:45:23AM +0530, Prithvi Tambewagh wrote:
> > > 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
> > >
> > > Hence ultimately, __configfs_open_file() was called, indirectly by
> > > target_core_item_dbroot_store(), and it also attempted to acquire
> > > &p->frag_sem, which was already held by the same thread, acquired earlier
> > > in flush_write_buffer. This poses a possibility of recursive locking,
> > > which triggers the lockdep warning.
> > >
> > > 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@...kaller.appspotmail.com
> > > Closes: https://syzkaller.appspot.com/bug?extid=f6e8174215573a84b797
> > > Tested-by: syzbot+f6e8174215573a84b797@...kaller.appspotmail.com
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Prithvi Tambewagh <activprithvi@...il.com>
> > > ---
> > >  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
> > >
> 
> You missed the very significant thing in the commit message - that this
> lockdep warning is due to try to write its own filename to dbroot file:
> 
> 	db_root: not a directory: /sys/kernel/config/target/dbroot
> 
> That is why the semaphore is the same - it is of the same file.
> 
> Without that explanation nobody understands wheter it is a false positive or not.
> 
> The fix itself looks good.
> 
> Reviewed-by: Dmitry Bogdanov <d.bogdanov@...ro.com> 

Hello Dmitry,

Thanks a lot for the feedback! I missed out this curcial explanation in the
commit message. I will update the commit message and send a v2 patch for 
the same.

Best Regards,
Prithvi