Re: [PATCH] crypto: testmgr - allocate buffers with __GFP_COMP

[Robin Murphy <robin.murphy@arm.com>]

On 17/04/2019 09:09, Russell King - ARM Linux admin wrote:
> On Tue, Apr 16, 2019 at 09:08:22PM -0700, Matthew Wilcox wrote:
>> On Mon, Apr 15, 2019 at 10:14:51PM -0500, Kees Cook wrote:
>>> On Mon, Apr 15, 2019 at 9:18 PM Matthew Wilcox <willy@infradead.org> wrote:
>>>> I agree; if the crypto code is never going to try to go from the address of
>>>> a byte in the allocation back to the head page, then there's no need to
>>>> specify GFP_COMP.
>>>>
>>>> But that leaves us in the awkward situation where
>>>> HARDENED_USERCOPY_PAGESPAN does need to be able to figure out whether
>>>> 'ptr + n - 1' lies within the same allocation as ptr.  Without using
>>>> a compound page, there's no indication in the VM structures that these
>>>> two pages were allocated as part of the same allocation.
>>>>
>>>> We could force all multi-page allocations to be compound pages if
>>>> HARDENED_USERCOPY_PAGESPAN is enabled, but I worry that could break
>>>> something.  We could make it catch fewer problems by succeeding if the
>>>> page is not compound.  I don't know, these all seem like bad choices
>>>> to me.
>>>
>>> If GFP_COMP is _not_ the correct signal about adjacent pages being
>>> part of the same allocation, then I agree: we need to drop this check
>>> entirely from PAGESPAN. Is there anything else that indicates this
>>> property? (Or where might we be able to store that info?)
>>
>> As far as I know, the page allocator does not store size information
>> anywhere, unless you use GFP_COMP.  That's why you have to pass
>> the 'order' to free_pages() and __free_pages().  It's also why
>> alloc_pages_exact() works (follow all the way into split_page()).
>>
>>> There are other pagespan checks, though, so those could stay. But I'd
>>> really love to gain page allocator allocation size checking ...
>>
>> I think that's a great idea, but I'm not sure how you'll be able to
>> do that.
> 
> However, we have had code (maybe historically now) that has allocated
> a higher order page and then handed back pages that it doesn't need -
> for example, when the code requires multiple contiguous pages but does
> not require a power-of-2 size of contiguous pages.

'git grep alloc_pages_exact' suggests it's not historical yet...

Robin.