Re: [PATCH 4/7] mm: Introduce verify_page_range()

[Peter Zijlstra <peterz@infradead.org>]

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.

The alternative is refactoring apply_to_page_range() :-/

---

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;
}

/**
 * verify_page_range() - Scan (and fill) a range of virtual memory and validate PTEs
 * @mm: mm identifying the virtual memory map
 * @addr: starting virtual address of the range
 * @size: size of the range
 * @fn: function that verifies the PTEs
 *
 * Scan a region of virtual memory, filling in page tables as necessary and
 * calling a provided function on each leaf, providing a copy of the
 * page-table-entry.
 *
 * Similar apply_to_page_range(), but does not provide direct access to the
 * page-tables.
 *
 * NOTE! this function does not work correctly vs large pages.
 *
 * Return: the address that failed verification or 0 on success.
 */
unsigned long verify_page_range(struct mm_struct *mm,
				unsigned long addr, unsigned long size,
				bool (*fn)(pte_t pte, unsigned long addr))
{
	struct vpr_data vpr = {
		.fn = fn,
		.addr = 0,
	};
	apply_to_page_range(mm, addr, size, vpr_fn, &vpr);
	return vpr.addr;
}
EXPORT_SYMBOL_GPL(verify_page_range);