From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B204CC48BCD for ; Wed, 9 Jun 2021 10:53:25 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 7D51261059 for ; Wed, 9 Jun 2021 10:53:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7D51261059 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:Cc:To:Subject:Reply-To:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=bUdz1OpO3FKJ8XvAAqV2JZgStkIlmrfrIYwnSk/mlnM=; b=AlxW71RFDJR96eIcRyTx8UjC7z B011cSkk3zdnGstbqB5IXh3mTIA7jZjbHjLrWH5Xl9NjdQAOirA6eksDk55XqWBLp4tCVfEs3pIT8 89krirn9UU+QUxK3siWGr5qvODRLU/Fy24HvgGpvFVwdLEm5f4OrNsrqklitqNDwYO+VggcHwVwuu oZ1PObRAyJIKbry0q0rWR8GO0HB/UPynlKyeVScd3VghlvtsXs2E+KKAjlJDp8Rz8h03w1HfR2IZF eQkJoQUsCyxD5SWTuJDd405zPbblm69+0AlpUfCBvcWAj/dBGUgIa96RyN0VgUdI43kpNH9ZmISLj nfDXr4tA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1lqvoF-00D8XO-4e; Wed, 09 Jun 2021 10:51:43 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1lqvoB-00D8WH-8J for linux-arm-kernel@lists.infradead.org; Wed, 09 Jun 2021 10:51:40 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 04FCC11B3; Wed, 9 Jun 2021 03:51:38 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5727F3F694; Wed, 9 Jun 2021 03:51:35 -0700 (PDT) Subject: Re: [PATCH v14 1/8] arm64: mte: Handle race when synchronising tags To: Marc Zyngier Cc: Catalin Marinas , 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 References: <20210607110816.25762-1-steven.price@arm.com> <20210607110816.25762-2-steven.price@arm.com> <875yynz5wp.wl-maz@kernel.org> From: Steven Price Message-ID: Date: Wed, 9 Jun 2021 11:51:34 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <875yynz5wp.wl-maz@kernel.org> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210609_035139_427151_33C8D865 X-CRM114-Status: GOOD ( 26.45 ) 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 09/06/2021 11:30, Marc Zyngier wrote: > On Mon, 07 Jun 2021 12:08:09 +0100, > Steven Price wrote: >> >> mte_sync_tags() used test_and_set_bit() to set the PG_mte_tagged flag >> before restoring/zeroing the MTE tags. However if another thread were to >> race and attempt to sync the tags on the same page before the first >> thread had completed restoring/zeroing then it would see the flag is >> already set and continue without waiting. This would potentially expose >> the previous contents of the tags to user space, and cause any updates >> that user space makes before the restoring/zeroing has completed to >> potentially be lost. >> >> Since this code is run from atomic contexts we can't just lock the page >> during the process. Instead implement a new (global) spinlock to protect >> the mte_sync_page_tags() function. >> >> Fixes: 34bfeea4a9e9 ("arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE") >> Reviewed-by: Catalin Marinas >> Signed-off-by: Steven Price >> --- >> arch/arm64/kernel/mte.c | 20 +++++++++++++++++--- >> 1 file changed, 17 insertions(+), 3 deletions(-) >> >> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c >> index 125a10e413e9..a3583a7fd400 100644 >> --- a/arch/arm64/kernel/mte.c >> +++ b/arch/arm64/kernel/mte.c >> @@ -25,6 +25,7 @@ >> u64 gcr_kernel_excl __ro_after_init; >> >> static bool report_fault_once = true; >> +static DEFINE_SPINLOCK(tag_sync_lock); >> >> #ifdef CONFIG_KASAN_HW_TAGS >> /* Whether the MTE asynchronous mode is enabled. */ >> @@ -34,13 +35,22 @@ EXPORT_SYMBOL_GPL(mte_async_mode); >> >> static void mte_sync_page_tags(struct page *page, pte_t *ptep, bool check_swap) >> { >> + unsigned long flags; >> pte_t old_pte = READ_ONCE(*ptep); >> >> + spin_lock_irqsave(&tag_sync_lock, flags); > > having though a bit more about this after an offline discussion with > Catalin: why can't this lock be made per mm? We can't really share > tags across processes anyway, so this is limited to threads from the > same process. Currently there's nothing stopping processes sharing tags (mmap(..., PROT_MTE, MAP_SHARED)) - I agree making use of this is tricky and it would have been nice if this had just been prevented from the beginning. Given the above, clearly the lock can't be per mm and robust. > I'd also like it to be documented that page sharing can only reliably > work with tagging if only one of the mappings is using tags. I'm not entirely clear whether you mean "can only reliably work" to be "is practically impossible to coordinate tag values", or whether you are proposing to (purposefully) introduce the race with a per-mm lock? (and document it). I guess we could have a per-mm lock and handle the race if user space screws up with the outcome being lost tags (double clear). But it feels to me like it could come back to bite in the future since VM_SHARED|VM_MTE will almost always work and I fear someone will start using it since it's permitted by the kernel. Steve _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel