Re: KASAN: use-after-free Read in link_path_walk

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Jul 24, 2018 at 06:17:26AM +0100, Al Viro wrote:

> Do we have LOOKUP_RCU in nd->flags at that point?  And how in hell
> did we get that dentry there?  In LOOKUP_RCU mode no freeing should
> be happening until after we call rcu_read_unlock(), unless the final
> dput() has happened before rcu_read_lock().  In which case we shouldn't
> have gotten to that dentry in the first place.

... except that we never set DCACHE_RCUACCESS for root dentry.  Which
invalidates the normal "if we run into dentry in lazy mode, its memory
won't be freed until we drop rcu_read_lock"...  d_make_root() definitely
needs to set DCACHE_RCUACCESS; whether it's all there is or you are
hitting something else is a separate question, of course...

>  And in non-LOOKUP_RCU
> mode we are bloody well holding references to everything (vfsmount
> and dentry alike), so that deactivate_super() shouldn't have been
> called as long as we are holding that reference.
> 
> Details, please.  Ideally - how to reproduce that.

Is there any way to tell KASAN that we want a crashdump triggered?
That would've been really useful for post-mortems...