From mboxrd@z Thu Jan 1 00:00:00 1970 From: Robin Murphy Subject: Re: [PATCH] arm64: kasan: fix phys_to_virt() false positive on tag-based kasan Date: Mon, 19 Aug 2019 15:35:16 +0100 Message-ID: <1ac7eb3e-156f-218c-8c5a-39a05dd46d55@arm.com> References: <20190819114420.2535-1-walter-zh.wu@mediatek.com> <20190819125625.bu3nbrldg7te5kwc@willie-the-truck> <20190819132347.GB9927@lakrids.cambridge.arm.com> <20190819133441.ejomv6cprdcz7hh6@willie-the-truck> <20190819142238.2jobs6vabkp2isg2@willie-the-truck> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20190819142238.2jobs6vabkp2isg2@willie-the-truck> Content-Language: en-GB Sender: linux-kernel-owner@vger.kernel.org To: Will Deacon , Andrey Konovalov Cc: Mark Rutland , Walter Wu , wsd_upstream@mediatek.com, Catalin Marinas , Will Deacon , LKML , kasan-dev , linux-mediatek@lists.infradead.org, Alexander Potapenko , Linux ARM , Matthias Brugger , Andrey Ryabinin , Andrew Morton , Dmitry Vyukov List-Id: linux-mediatek@lists.infradead.org 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 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). Robin.