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

From: Robin Murphy
Date: Wed Apr 17 2019 - 05:54:45 EST

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@xxxxxxxxxxxxx> 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...