linux-kernel - Re: [syzbot] [fs?] KASAN: slab-use-after-free Read in test_bdev_super

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Date:   Fri, 4 Aug 2023 15:20:45 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     syzbot <syzbot+2faac0423fdc9692822b@...kaller.appspotmail.com>,
        jack@...e.cz, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        viro@...iv.linux.org.uk
Subject: Re: [syzbot] [fs?] KASAN: slab-use-after-free Read in
 test_bdev_super_fc

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?