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. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up
