linux-kernel - Re: [PATCH 4/7] mm: Introduce verify_page

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Date:   Tue, 13 Apr 2021 20:01:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, x86@...nel.org,
        jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, chris@...is-wilson.co.uk,
        intel-gfx@...ts.freedesktop.org, linux-mm@...ck.org, hch@....de
Subject: Re: [PATCH 4/7] mm: Introduce verify_page_range()

On Tue, Apr 13, 2021 at 09:36:32AM +0200, Peter Zijlstra wrote:
> On Mon, Apr 12, 2021 at 01:05:09PM -0700, Kees Cook wrote:
> > On Mon, Apr 12, 2021 at 10:00:16AM +0200, Peter Zijlstra wrote:
> > > +struct vpr_data {
> > > +	int (*fn)(pte_t pte, unsigned long addr, void *data);
> > > +	void *data;
> > > +};
> > 
> > Eeerg. This is likely to become an attack target itself. Stored function
> > pointer with stored (3rd) argument.
> > 
> > This doesn't seem needed: only DRM uses it, and that's for error
> > reporting. I'd rather plumb back errors in a way to not have to add
> > another place in the kernel where we do func+arg stored calling.
> 
> Is this any better? It does have the stored pointer, but not a stored
> argument, assuming you don't count returns as arguments I suppose.

It's better in the sense that it's not the func/arg pair that really
bugs me, yes. :)

> The alternative is refactoring apply_to_page_range() :-/

Yeah, I'm looking now, I see what you mean.

> ---
> 
> struct vpr_data {
> 	bool (*fn)(pte_t pte, unsigned long addr);
> 	unsigned long addr;
> };
> 
> static int vpr_fn(pte_t *pte, unsigned long addr, void *data)
> {
> 	struct vpr_data *vpr = data;
> 	if (!vpr->fn(*pte, addr)) {
> 		vpr->addr = addr;
> 		return -EINVAL;
> 	}
> 	return 0;
> }

My point about passing "addr" was that nothing in the callback actually
needs it -- the top level can just as easily report the error. And that
the helper is always vpr_fn(), so it doesn't need to be passed either.

So the addr can just be encoded in "int", and no structure is needed at:

typedef bool (*vpr_fn_t)(pte_t pte);

static int vpr_fn(pte_t *pte, unsigned long addr, void *data)
{
	vpr_fn_t callback = data;

	if (!callback(*pte))
		return addr >> PAGE_SIZE;
	return 0;
}

unsigned long verify_page_range(struct mm_struct *mm,
				unsigned long addr, unsigned long size,
				vpr_fn_t callback)
{
	return apply_to_page_range(mm, addr, size, vpr_fn, callback) << PAGE_SIZE;
}

But maybe I'm missing something?

-- 
Kees Cook