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Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.94 #2 (Red Hat Linux)) id 1ljhkT-00GJgh-3p for linux-arm-kernel@lists.infradead.org; Thu, 20 May 2021 12:25:59 +0000 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5391A6124C; Thu, 20 May 2021 12:25:53 +0000 (UTC) Date: Thu, 20 May 2021 13:25:50 +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 3/8] arm64: mte: Sync tags for pages where PTE is untagged Message-ID: <20210520122550.GD12251@arm.com> References: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-4-steven.price@arm.com> <20210519180610.GE21619@arm.com> <3bac3a47-9f96-c7bf-e401-fdef60dcc9d8@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <3bac3a47-9f96-c7bf-e401-fdef60dcc9d8@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_052557_222728_D1108449 X-CRM114-Status: GOOD ( 41.08 ) 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 12:55:21PM +0100, Steven Price wrote: > On 19/05/2021 19:06, Catalin Marinas wrote: > > On Mon, May 17, 2021 at 01:32:34PM +0100, Steven Price wrote: > >> A KVM guest could store tags in a page even if the VMM hasn't mapped > >> the page with PROT_MTE. So when restoring pages from swap we will > >> need to check to see if there are any saved tags even if !pte_tagged(). > >> > >> However don't check pages for which pte_access_permitted() returns false > >> as these will not have been swapped out. > >> > >> Signed-off-by: Steven Price > >> --- > >> arch/arm64/include/asm/pgtable.h | 9 +++++++-- > >> arch/arm64/kernel/mte.c | 16 ++++++++++++++-- > >> 2 files changed, 21 insertions(+), 4 deletions(-) > >> > >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > >> index 0b10204e72fc..275178a810c1 100644 > >> --- a/arch/arm64/include/asm/pgtable.h > >> +++ b/arch/arm64/include/asm/pgtable.h > >> @@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, > >> if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte)) > >> __sync_icache_dcache(pte); > >> > >> - if (system_supports_mte() && > >> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte)) > >> + /* > >> + * If the PTE would provide user space access to the tags associated > >> + * with it then ensure that the MTE tags are synchronised. Exec-only > >> + * mappings don't expose tags (instruction fetches don't check tags). > >> + */ > >> + if (system_supports_mte() && pte_present(pte) && > >> + pte_access_permitted(pte, false) && !pte_special(pte)) > >> mte_sync_tags(ptep, pte); > > > > Looking at the mte_sync_page_tags() logic, we bail out early if it's the > > old pte is not a swap one and the new pte is not tagged. So we only need > > to call mte_sync_tags() if it's a tagged new pte or the old one is swap. > > What about changing the set_pte_at() test to: > > > > if (system_supports_mte() && pte_present(pte) && !pte_special(pte) && > > (pte_tagged(pte) || is_swap_pte(READ_ONCE(*ptep)))) > > mte_sync_tags(ptep, pte); > > > > We can even change mte_sync_tags() to take the old pte directly: > > > > if (system_supports_mte() && pte_present(pte) && !pte_special(pte)) { > > pte_t old_pte = READ_ONCE(*ptep); > > if (pte_tagged(pte) || is_swap_pte(old_pte)) > > mte_sync_tags(old_pte, pte); > > } > > > > It would save a function call in most cases where the page is not > > tagged. > > Yes that looks like a good optimisation - although you've missed the > pte_access_permitted() part of the check ;) I was actually wondering if we could remove it. I don't think it buys us much as we have a pte_present() check already, so we know it is pointing to a valid page. Currently we'd only get a tagged pte on user mappings, same with swap entries. When vmalloc kasan_hw will be added, I think we have a set_pte_at() with a tagged pte but init_mm and high address (we might as well add a warning if addr > TASK_SIZE_64 on the mte_sync_tags path so that we don't forget). > The problem I hit is one of include dependencies: > > is_swap_pte() is defined (as a static inline) in > include/linux/swapops.h. However the definition depends on > pte_none()/pte_present() which are defined in pgtable.h - so there's a > circular dependency. > > Open coding is_swap_pte() in set_pte_at() works, but it's a bit ugly. > Any ideas on how to improve on the below? > > if (system_supports_mte() && pte_present(pte) && > pte_access_permitted(pte, false) && !pte_special(pte)) { > pte_t old_pte = READ_ONCE(*ptep); > /* > * We only need to synchronise if the new PTE has tags enabled > * or if swapping in (in which case another mapping may have > * set tags in the past even if this PTE isn't tagged). > * (!pte_none() && !pte_present()) is an open coded version of > * is_swap_pte() > */ > if (pte_tagged(pte) || (!pte_none(pte) && !pte_present(pte))) > mte_sync_tags(old_pte, pte); > } That's why I avoided testing my suggestion ;). I think we should just add !pte_none() in there with a comment that it may be a swap pte and use the is_swap_pte() again on the mte_sync_tags() path. We already have the pte_present() check. -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel