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Date:   Fri, 4 Aug 2023 16:02:01 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Christian Brauner <brauner@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>,
        syzbot <syzbot+2faac0423fdc9692822b@syzkaller.appspotmail.com>,
        jack@suse.cz, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        viro@zeniv.linux.org.uk
Subject: Re: [syzbot] [fs?] KASAN: slab-use-after-free Read in
 test_bdev_super_fc
Message-ID: <20230804140201.GA27600@lst.de>
References: <00000000000058d58e06020c1cab@google.com> <20230804101408.GA23274@lst.de> <20230804-abstieg-behilflich-eda2ce9c2c0f@brauner>
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On Fri, Aug 04, 2023 at 03:20:45PM +0200, Christian Brauner wrote:
> On Fri, Aug 04, 2023 at 12:14:08PM +0200, Christoph Hellwig wrote:
> > FYI, I can reproduce this trivially locally, but even after spending a
> > significant time with the trace I'm still puzzled at what is going
> > on.  I've started trying to make sense of the lockdep report about
> > returning to userspace with s_umount held, originall locked in
> > get_tree_bdev and am still missing how it could happen.
> 
> So in the old scheme:
> 
> s = alloc_super()
> -> down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
> 
> and assume you're not finding an old one immediately afterwards you'd
> 
> -> spin_lock(&sb_lock)
> 
> static int set_bdev_super(struct super_block *s, void *data)
> {
>         s->s_bdev = data;
>         s->s_dev = s->s_bdev->bd_dev;
>         s->s_bdi = bdi_get(s->s_bdev->bd_disk->bdi);
> 
>         if (bdev_stable_writes(s->s_bdev))
>                 s->s_iflags |= SB_I_STABLE_WRITES;
>         return 0;
> }
> 
> -> spin_unlock(&sb_lock)
> 
> in the new scheme you're doing:
> 
> s = alloc_super()
> -> down_write_nested(&s->s_umount, SINGLE_DEPTH_NESTING);
> 
> and assume you're not finding an old one immediately afterwards you'd
> 
> up_write(&s->s_umount);
> 
> error = setup_bdev_super(s, fc->sb_flags, fc);
> -> spin_lock(&sb_lock);
>    sb->s_bdev = bdev;
>    sb->s_bdi = bdi_get(bdev->bd_disk->bdi);
>    if (bdev_stable_writes(bdev))
>            sb->s_iflags |= SB_I_STABLE_WRITES;
> -> spin_unlock(&sb_lock);
> 
> down_write(&s->s_umount);
> 
> Which looks like the lock ordering here is changed?

Yes, that none only should be safe, but more importantly should not
lead to a return to userspace with s_umount held.

Anyway, debugging a regression in mainline right now so I'm taking a
break from this one.