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

[Matthew Wilcox]

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.