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Thu, 20 May 2021 17:50:46 +0000 Received: by mail.kernel.org (Postfix) with ESMTPSA id A7A56613E8; Thu, 20 May 2021 17:50:40 +0000 (UTC) Date: Thu, 20 May 2021 18:50:38 +0100 From: Catalin Marinas To: Steven Price Cc: Marc Zyngier , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones Subject: Re: [PATCH v12 4/8] arm64: kvm: Introduce MTE VM feature Message-ID: <20210520175037.GG12251@arm.com> References: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-5-steven.price@arm.com> <20210520115426.GB12251@arm.com> <5f0996d6-0a6e-ebcd-afcd-8290faba6780@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <5f0996d6-0a6e-ebcd-afcd-8290faba6780@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210520_105044_555474_0AA22213 X-CRM114-Status: GOOD ( 37.90 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, May 20, 2021 at 04:05:46PM +0100, Steven Price wrote: > On 20/05/2021 12:54, Catalin Marinas wrote: > > On Mon, May 17, 2021 at 01:32:35PM +0100, Steven Price wrote: > >> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > >> index c5d1f3c87dbd..8660f6a03f51 100644 > >> --- a/arch/arm64/kvm/mmu.c > >> +++ b/arch/arm64/kvm/mmu.c > >> @@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot, > >> return PAGE_SIZE; > >> } > >> > >> +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size, > >> + kvm_pfn_t pfn) > >> +{ > >> + if (kvm_has_mte(kvm)) { > >> + /* > >> + * The page will be mapped in stage 2 as Normal Cacheable, so > >> + * the VM will be able to see the page's tags and therefore > >> + * they must be initialised first. If PG_mte_tagged is set, > >> + * tags have already been initialised. > >> + */ > >> + unsigned long i, nr_pages = size >> PAGE_SHIFT; > >> + struct page *page = pfn_to_online_page(pfn); > >> + > >> + if (!page) > >> + return -EFAULT; > > > > IIRC we ended up with pfn_to_online_page() to reject ZONE_DEVICE pages > > that may be mapped into a guest and we have no idea whether they support > > MTE. It may be worth adding a comment, otherwise, as Marc said, the page > > wouldn't disappear. > > I'll add a comment. > > >> + > >> + for (i = 0; i < nr_pages; i++, page++) { > >> + if (!test_and_set_bit(PG_mte_tagged, &page->flags)) > >> + mte_clear_page_tags(page_address(page)); > > > > We started the page->flags thread and ended up fixing it for the host > > set_pte_at() as per the first patch: > > > > https://lore.kernel.org/r/c3293d47-a5f2-ea4a-6730-f5cae26d8a7e@arm.com > > > > Now, can we have a race between the stage 2 kvm_set_spte_gfn() and a > > stage 1 set_pte_at()? Only the latter takes a lock. Or between two > > kvm_set_spte_gfn() in different VMs? I think in the above thread we > > concluded that there's only a problem if the page is shared between > > multiple VMMs (MAP_SHARED). How realistic is this and what's the > > workaround? > > > > Either way, I think it's worth adding a comment here on the race on > > page->flags as it looks strange that here it's just a test_and_set_bit() > > while set_pte_at() uses a spinlock. > > > > Very good point! I should have thought about that. I think splitting the > test_and_set_bit() in two (as with the cache flush) is sufficient. While > there technically still is a race which could lead to user space tags > being clobbered: > > a) It's very odd for a VMM to be doing an mprotect() after the fact to > add PROT_MTE, or to be sharing the memory with another process which > sets PROT_MTE. > > b) The window for the race is incredibly small and the VMM (generally) > needs to be robust against the guest changing tags anyway. > > But I'll add a comment here as well: > > /* > * There is a potential race between sanitising the > * flags here and user space using mprotect() to add > * PROT_MTE to access the tags, however by splitting > * the test/set the only risk is user space tags > * being overwritten by the mte_clear_page_tags() call. > */ I think (well, I haven't re-checked), an mprotect() in the VMM ends up calling set_pte_at_notify() which would call kvm_set_spte_gfn() and that will map the page in the guest. So the problem only appears between different VMMs sharing the same page. In principle they can be MAP_PRIVATE but they'd be CoW so the race wouldn't matter. So it's left with MAP_SHARED between multiple VMMs. I think we should just state that this is unsafe and they can delete each-others tags. If we are really worried, we can export that lock you added in mte.c. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel