linux-kernel - Re: KASAN: use-after-free Read in link_path

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Message-ID: <20180806130147.GL15082@ZenIV.linux.org.uk>
Date:   Mon, 6 Aug 2018 14:01:47 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     "Dae R. Jeong" <threeearcat@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        byoungyoung@...due.edu, kt0755@...il.com, bammanag@...due.edu
Subject: Re: KASAN: use-after-free Read in link_path_walk

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...