Subject: Re: [PATCH] vmap(): don't allow invalid pages

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 19 Jan 2022 19:12:22 +0000

On Wed, Jan 19, 2022 at 06:43:10PM +0000, Robin Murphy wrote:
> Indeed, my impression is that the only legitimate way to get hold of a page
> pointer without assumed provenance is via pfn_to_page(), which is where
> pfn_valid() comes in. Thus pfn_valid(page_to_pfn()) really *should* be a
> tautology.

That can only be true if pfn == page_to_pfn(pfn_to_page(pfn)) for all
values of pfn.

Given how pfn_to_page() is defined in the sparsemem case:

#define __pfn_to_page(pfn)                              \
({	unsigned long __pfn = (pfn);                    \
	struct mem_section *__sec = __pfn_to_section(__pfn);    \
	__section_mem_map_addr(__sec) + __pfn;          \
})
#define page_to_pfn __page_to_pfn

that isn't the case, especially when looking at page_to_pfn():

#define __page_to_pfn(pg)                                       \
({      const struct page *__pg = (pg);                         \
        int __sec = page_to_section(__pg);                      \
	(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec))); \
})

Where:

static inline unsigned long page_to_section(const struct page *page)
{
	return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
}

So if page_to_section() returns something that is, e.g. zero for an
invalid page in a non-zero section, you're not going to end up with
the right pfn from page_to_pfn().

As I've said now a couple of times, trying to determine of a struct
page pointer is valid is the wrong question to be asking.

-- 
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