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

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Date:   Thu, 31 Jan 2019 23:09:11 +1300
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Omar Sandoval <osandov@...com>,
        syzbot <syzbot+b382ba6a802a3d242790@...kaller.appspotmail.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...com>
Subject: Re: BUG: unable to handle kernel paging request in dput (2)

On Thu, Jan 31, 2019 at 12:41 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> 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
>

FYI, this patch does not fix the relay.c crash I bisected... I think
more clean-up is needed?

-Kees