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

[Russell King - ARM Linux admin]

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.

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