Messages in this thread | | | Date | Sat, 11 May 2019 21:11:42 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH] usercopy: Remove HARDENED_USERCOPY_PAGESPAN |
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On Sat, May 11, 2019 at 05:03:08PM -0700, Kees Cook wrote: > On Fri, May 10, 2019 at 08:41:43PM -0400, Laura Abbott wrote: > > On 5/10/19 3:43 PM, Kees Cook wrote: > > > This feature continues to cause more problems than it solves[1]. Its > > > intention was to check the bounds of page-allocator allocations by using > > > __GFP_COMP, for which we would need to find all missing __GFP_COMP > > > markings. This work has been on hold and there is an argument[2] > > > that such markings are not even the correct signal for checking for > > > same-allocation pages. Instead of depending on BROKEN, this just removes > > > it entirely. It can be trivially reverted if/when a better solution for > > > tracking page allocator sizes is found. > > > > > > [1] https://www.mail-archive.com/linux-crypto@vger.kernel.org/msg37479.html > > > [2] https://lkml.kernel.org/r/20190415022412.GA29714@bombadil.infradead.org > > > > I agree the page spanning is broken but is it worth keeping the > > checks against __rodata __bss etc.? > > They're all just white-listing later checks (except RODATA which is > doing a cheap RO test which is redundant on any architecture with actual > rodata...) so they don't have any value in staying without the rest of > the page allocator logic. > > > > - /* Is the object wholly within one base page? */ > > > - if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) == > > > - ((unsigned long)end & (unsigned long)PAGE_MASK))) > > > - return; > > > - > > > - /* Allow if fully inside the same compound (__GFP_COMP) page. */ > > > - endpage = virt_to_head_page(end); > > > - if (likely(endpage == page)) > > > - return; > > We _could_ keep the mixed CMA/reserved/neither check if we really wanted > to, but that's such a corner case of a corner case, I'm not sure it's > worth doing the virt_to_head_page() across the whole span to figure > it out.
I'd delete that first check, because it's a subset of the second check,
/* Is the object wholly within a single (base or compound) page? */ endpage = virt_to_head_page(end); if (likely(endpage == page)) return;
/* * If the start and end are more than MAX_ORDER apart, they must * be from separate allocations */ if (n >= (PAGE_SIZE << MAX_ORDER)) usercopy_abort("spans too many pages", NULL, to_user, 0, n);
/* * If neither page is compound, we can't tell if the object is * within a single allocation or not */ if (!PageCompound(page) && !PageCompound(endpage)) return;
> I really wish we had size of allocation reliably held somewhere. We'll > need it for doing memory tagging of the page allocator too...
I think we'll need to store those allocations in a separate data structure on the side. As far as the rest of the kernel is concerned, those struct pages belong to them once the page allocator has given them.
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