Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze_sb

[Christoph Hellwig]

On Thu, Jan 07, 2021 at 11:08:39PM +0000, Satya Tangirala wrote:
> >  		error = sb->s_op->freeze_super(sb);
> >  	else
> > @@ -600,6 +602,7 @@ int thaw_bdev(struct block_device *bdev)
> >  	if (!sb)
> >  		goto out;
> >  
> > +	bdev->bd_fsfreeze_sb = NULL;
> This causes bdev->bd_fsfreeze_sb to be set to NULL even if the call to
> thaw_super right after this line fail. So if a caller tries to call
> thaw_bdev() again after receiving such an error, that next call won't even
> try to call thaw_super(). Is that what we want here?  (I don't know much
> about this code, but from a cursory glance I think this difference is
> visible to emergency_thaw_bdev() in fs/buffer.c)

Yes, that definitively is an issue.

> 
> I think the second difference (decrementing bd_fsfreeze_count when
> get_active_super() returns NULL) doesn't change anything w.r.t the
> use-after-free. It does however, change the behaviour of the function
> slightly, and it might be caller visible (because from a cursory glance, it
> looks like we're reading the bd_fsfreeze_count from some other places like
> fs/super.c). Even before 040f04bd2e82, the code wouldn't decrement
> bd_fsfreeze_count when get_active_super() returned NULL - so is this change
> in behaviour intentional? And if so, maybe it should go in a separate
> patch?

Yes, that would be a change in behavior.  And I'm not sure why we would
want to change it.  But if so we should do it in a separate patch that
documents the why, on top of the patch that already is in the block tree.