linux-kernel - Re: BUG: unable to handle kernel paging request in dput (2)

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Date:   Wed, 30 Jan 2019 12:40:52 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Omar Sandoval <osandov@...com>,
        syzbot <syzbot+b382ba6a802a3d242790@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        Jens Axboe <axboe@...com>
Subject: Re: BUG: unable to handle kernel paging request in dput (2)

On Wed, Jan 30, 2019 at 08:26:24PM +0900, Tetsuo Handa wrote:
> On 2019/01/30 20:11, Tetsuo Handa wrote:
> > Hello, Omar.
> > 
> > syzbot is reporting a crash due to dput(-EINVAL) [1]. I think the location is
> > 
> >         dir = debugfs_lookup(buts->name, blk_debugfs_root);
> >         if (!dir)
> >                 bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
> > 
> > added by commit 6ac93117ab009d39 ("blktrace: use existing disk debugfs directory").
> > 
> > Currently, Greg Kroah-Hartman is posting patches:
> > 
> >   When calling debugfs functions, there is no need to ever check the
> >   return value.  The function can work or not, but the code logic should
> >   never do something different based on this.
> > 
> > Omar, what do you want to do for this case?
> > 
> > [1] https://syzkaller.appspot.com/bug?extid=b382ba6a802a3d242790
> > 
> 
> The function which returned -EINVAL instead of NULL seems to be debugfs_lookup()
> modified by commit ff9fb72bc07705c0 ("debugfs: return error values, not NULL").

Ok, the patch below should fix this up.

thanks,

greg k-h

-------------------------

From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH] debugfs: debugfs_lookup() should return NULL if not found

Lots of callers of debugfs_lookup() were just checking NULL to see if
the file/directory was found or not.  By changing this in ff9fb72bc077
("debugfs: return error values, not NULL") we caused some subsystems to
easily crash.

Fixes: ff9fb72bc077 ("debugfs: return error values, not NULL")
Reported-by: syzbot+b382ba6a802a3d242790@...kaller.appspotmail.com
Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: Omar Sandoval <osandov@...com>
Cc: Jens Axboe <axboe@...com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/debugfs/inode.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index b16f8035b1af..29c68c5d44d5 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -254,8 +254,8 @@ MODULE_ALIAS_FS("debugfs");
  * @parent: a pointer to the parent dentry of the file.
  *
  * This function will return a pointer to a dentry if it succeeds.  If the file
- * doesn't exist or an error occurs, %ERR_PTR(-ERROR) will be returned.  The
- * returned dentry must be passed to dput() when it is no longer needed.
+ * doesn't exist or an error occurs, %NULL will be returned.  The returned
+ * dentry must be passed to dput() when it is no longer needed.
  *
  * If debugfs is not enabled in the kernel, the value -%ENODEV will be
  * returned.
@@ -265,17 +265,17 @@ struct dentry *debugfs_lookup(const char *name, struct dentry *parent)
 	struct dentry *dentry;
 
 	if (IS_ERR(parent))
-		return parent;
+		return NULL;
 
 	if (!parent)
 		parent = debugfs_mount->mnt_root;
 
 	dentry = lookup_one_len_unlocked(name, parent, strlen(name));
 	if (IS_ERR(dentry))
-		return dentry;
+		return NULL;
 	if (!d_really_is_positive(dentry)) {
 		dput(dentry);
-		return ERR_PTR(-EINVAL);
+		return NULL;
 	}
 	return dentry;
 }
-- 
2.20.1