linux-hardening - Re: [PATCH 1/3] usercopy: Handle vm_map

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Message-ID: <Yqdtkhi+AAejtekZ@pc638.lan>
Date:   Mon, 13 Jun 2022 19:02:10 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        Uladzislau Rezki <urezki@...il.com>,
        Zorro Lang <zlang@...hat.com>, linux-xfs@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/3] usercopy: Handle vm_map_ram() areas

On Mon, Jun 13, 2022 at 05:44:35PM +0100, Matthew Wilcox wrote:
> On Mon, Jun 13, 2022 at 09:23:15AM -0700, Kees Cook wrote:
> > On Sun, Jun 12, 2022 at 10:32:25PM +0100, Matthew Wilcox (Oracle) wrote:
> > > vmalloc does not allocate a vm_struct for vm_map_ram() areas.  That causes
> > > us to deny usercopies from those areas.  This affects XFS which uses
> > > vm_map_ram() for its directories.
> > > 
> > > Fix this by calling find_vmap_area() instead of find_vm_area().
> > 
> > Thanks for the fixes!
> > 
> > > [...]
> > > +		/* XXX: We should also abort for free vmap_areas */
> > 
> > What's needed to detect this?
> 
> I'm not entirely sure.  I only just learned of the existence of this
> struct ;-)
> 
>         /*
>          * The following two variables can be packed, because
>          * a vmap_area object can be either:
>          *    1) in "free" tree (root is free_vmap_area_root)
>          *    2) or "busy" tree (root is vmap_area_root)
>          */
>         union {
>                 unsigned long subtree_max_size; /* in "free" tree */
>                 struct vm_struct *vm;           /* in "busy" tree */
>         };
> 
> Hmm.  Actually, we only search vmap_area_root, so I suppose it can't
> be a free area.  So this XXX can be removed, as we'll get NULL back
> if we've got a pointer to a free area.  Ulad, do I have this right?
>
Yep, we find here only allocated areas which bind to the "vmap_area_root"
tree, so it can not be a freed area. So we will not get a pointer to the
free area :)

--
Uladzislau Rezki