Messages in this thread | | | Date | Mon, 19 Aug 2019 15:43:55 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on tag-based kasan |
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On Mon, Aug 19, 2019 at 03:35:16PM +0100, Robin Murphy wrote: > On 19/08/2019 15:22, Will Deacon wrote: > > On Mon, Aug 19, 2019 at 04:05:22PM +0200, Andrey Konovalov wrote: > > > On Mon, Aug 19, 2019 at 3:34 PM Will Deacon <will@kernel.org> wrote: > > > > > > > > On Mon, Aug 19, 2019 at 02:23:48PM +0100, Mark Rutland wrote: > > > > > On Mon, Aug 19, 2019 at 01:56:26PM +0100, Will Deacon wrote: > > > > > > On Mon, Aug 19, 2019 at 07:44:20PM +0800, Walter Wu wrote: > > > > > > > __arm_v7s_unmap() call iopte_deref() to translate pyh_to_virt address, > > > > > > > but it will modify pointer tag into 0xff, so there is a false positive. > > > > > > > > > > > > > > When enable tag-based kasan, phys_to_virt() function need to rewrite > > > > > > > its original pointer tag in order to avoid kasan report an incorrect > > > > > > > memory corruption. > > > > > > > > > > > > Hmm. Which tree did you see this on? We've recently queued a load of fixes > > > > > > in this area, but I /thought/ they were only needed after the support for > > > > > > 52-bit virtual addressing in the kernel. > > > > > > > > > > I'm seeing similar issues in the virtio blk code (splat below), atop of > > > > > the arm64 for-next/core branch. I think this is a latent issue, and > > > > > people are only just starting to test with KASAN_SW_TAGS. > > > > > > > > > > It looks like the virtio blk code will round-trip a SLUB-allocated pointer from > > > > > virt->page->virt, losing the per-object tag in the process. > > > > > > > > > > Our page_to_virt() seems to get a per-page tag, but this only makes > > > > > sense if you're dealing with the page allocator, rather than something > > > > > like SLUB which carves a page into smaller objects giving each object a > > > > > distinct tag. > > > > > > > > > > Any round-trip of a pointer from SLUB is going to lose the per-object > > > > > tag. > > > > > > > > Urgh, I wonder how this is supposed to work? > > > > > > > > If we end up having to check the KASAN shadow for *_to_virt(), then why > > > > do we need to store anything in the page flags at all? Andrey? > > > > > > As per 2813b9c0 ("kasan, mm, arm64: tag non slab memory allocated via > > > pagealloc") we should only save a non-0xff tag in page flags for non > > > slab pages. > > > > Thanks, that makes sense. Hopefully the patch from Andrey R will solve > > both of the reported splats, since I'd not realised they were both on the > > kfree() path. > > > > > Could you share your .config so I can reproduce this? > > > > This is in the iopgtable code, so it's probably pretty tricky to trigger > > at runtime unless you have the write IOMMU hardware, unfortunately. > > If simply freeing any entry from the l2_tables cache is sufficient, then the > short-descriptor selftest should do the job, and that ought to run on > anything (modulo insane RAM layouts).
Ok, so that would be defconfig + CONFIG_IOMMU_IO_PGTABLE_ARMV7S + CONFIG_IOMMU_IO_PGTABLE_ARMV7S_SELFTEST + KASAN...
Will
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