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

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Date:   Mon, 13 Jun 2022 17:44:35 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     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 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?