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

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Date:   Fri, 4 Aug 2023 16:36:49 +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 04:02:01PM +0200, Christoph Hellwig wrote:
> 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.

FFS

diff --git a/fs/romfs/super.c b/fs/romfs/super.c
index c59b230d55b4..96023fac1ed8 100644
--- a/fs/romfs/super.c
+++ b/fs/romfs/super.c
@@ -590,10 +590,7 @@ static void romfs_kill_sb(struct super_block *sb)
        }
 #endif
 #ifdef CONFIG_ROMFS_ON_BLOCK
-       if (sb->s_bdev) {
-               kill_block_super(sb);
-               return;
-       }
+       kill_block_super(sb);
 #endif
 }