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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id E1C89CAC5B0 for ; Wed, 24 Sep 2025 12:48:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=csGzN7ITFwXhWMh/RaKjq9M87J1ZtN3tzt91FwMGJYc=; b=pG7n0DYfueU7PhFcX1AUKJeKRu /h8wror4RadYXJ1uMsnFmg5VtN0u9TYfF0haRnloF9eExXyGjR6pARrWAL1FPAmffCVOoUIVZ8D9z VgPRIbkgjBg6cCuqbDc0WJEbHEPXogq3ygAMMlUVXWfgZZC6WWDwLV+7RsOwyy81jMwjtQbxkpXCq wzfDrFPuWyFpntj2+zMs+FNGt8848+Gaa4I90mfQP21eCTuAK7K/D2s//M9lRFnbNQi98u1NhLni2 alwXqlRwgZMGquPbZSAr8YE5WloW110gkFZgE1yVq2X6SuPGwh8fdVCvMRynxVUEqKQzTBo6uXSQJ d+sHcJMw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1v1OvE-0000000HXh8-0gtp; Wed, 24 Sep 2025 12:48:36 +0000 Received: from out-174.mta0.migadu.com ([91.218.175.174]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1v1OvB-0000000HXdU-0D5U for linux-arm-kernel@lists.infradead.org; Wed, 24 Sep 2025 12:48:34 +0000 Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1758718108; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=csGzN7ITFwXhWMh/RaKjq9M87J1ZtN3tzt91FwMGJYc=; b=IfoHwq/IQBZPRqQ4ISbsT/VJvaDGntnyP7fTCR8NlNmtCkQd4wXYZQFC0/Fm2GwRJ2f/tv urWjCgrciJGntAq2OwXCmTXpg21vW7XrF2SjIHc22X9l8wWL6cH/CUhzcHzBKVwH7hSXeH eDqHQ2kuv7+qScnJhzjnlhG6y6BhmSA= Date: Wed, 24 Sep 2025 20:48:18 +0800 MIME-Version: 1.0 Subject: Re: [PATCH] arm64: mte: Do not flag the zero page as PG_mte_tagged Content-Language: en-US To: Catalin Marinas Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, Gergely Kovacs , David Hildenbrand , Will Deacon References: <20250924123528.1536835-1-catalin.marinas@arm.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <20250924123528.1536835-1-catalin.marinas@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250924_054833_393960_0D3F83A7 X-CRM114-Status: GOOD ( 30.04 ) 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: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 2025/9/24 20:31, Catalin Marinas wrote: > Commit 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to the > zero page") attempted to fix ptrace() reading of tags from the zero page > by marking it as PG_mte_tagged during cpu_enable_mte(). The same commit > also changed the ptrace() tag access permission check to the VM_MTE vma > flag while turning the page flag test into a WARN_ON_ONCE(). > > Attempting to set the PG_mte_tagged flag early with > CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled may either hang (after commit > d77e59a8fccd "arm64: mte: Lock a page for MTE tag initialisation") or > have the flags cleared later during page_alloc_init_late(). In addition, > pages_identical() -> memcmp_pages() will reject any comparison with the > zero page as it is marked as tagged. > > Partially revert the above commit to avoid setting PG_mte_tagged on the > zero page. Update the __access_remote_tags() warning on untagged pages > to ignore the zero page since it is known to have the tags initialised. > > Note that all user mapping of the zero page are marked as pte_special(). > The arm64 set_pte_at() will not call mte_sync_tags() on such pages, so > PG_mte_tagged will remain cleared. > > Signed-off-by: Catalin Marinas > Fixes: 68d54ceeec0e ("arm64: mte: Allow PTRACE_PEEKMTETAGS access to the zero page") > Reported-by: Gergely Kovacs > Cc: # 5.10.x > Cc: Will Deacon > Cc: David Hildenbrand > Cc: Lance Yang > --- > > For reference, discussion on page merging here: > > https://lore.kernel.org/r/aNKJ5glToE4hMhWA@arm.com Cool. LGTM. Acked-by: Lance Yang > > The deferred struct page init problem was reported by Gergely offline. > > Given that we've had this bug for over four years and it was only > recently noticed, I think we should merge it at -rc1, give it a bit more > time in -next in case it breaks anything (not likely but you never know, > MTE interaction with the mm code is always surprising ;)). > > arch/arm64/kernel/cpufeature.c | 10 +++++++--- > arch/arm64/kernel/mte.c | 2 +- > 2 files changed, 8 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c > index ef269a5a37e1..3e9d1aa37bbf 100644 > --- a/arch/arm64/kernel/cpufeature.c > +++ b/arch/arm64/kernel/cpufeature.c > @@ -2408,17 +2408,21 @@ static void bti_enable(const struct arm64_cpu_capabilities *__unused) > #ifdef CONFIG_ARM64_MTE > static void cpu_enable_mte(struct arm64_cpu_capabilities const *cap) > { > + static bool cleared_zero_page = false; > + > sysreg_clear_set(sctlr_el1, 0, SCTLR_ELx_ATA | SCTLR_EL1_ATA0); > > mte_cpu_setup(); > > /* > * Clear the tags in the zero page. This needs to be done via the > - * linear map which has the Tagged attribute. > + * linear map which has the Tagged attribute. Since this page is > + * always mapped as pte_special(), set_pte_at() will not attempt to > + * clear the tags or set PG_mte_tagged. > */ > - if (try_page_mte_tagging(ZERO_PAGE(0))) { > + if (!cleared_zero_page) { > + cleared_zero_page = true; > mte_clear_page_tags(lm_alias(empty_zero_page)); > - set_page_mte_tagged(ZERO_PAGE(0)); > } > > kasan_init_hw_tags_cpu(); > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c > index e5e773844889..63aed49ac181 100644 > --- a/arch/arm64/kernel/mte.c > +++ b/arch/arm64/kernel/mte.c > @@ -460,7 +460,7 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr, > if (folio_test_hugetlb(folio)) > WARN_ON_ONCE(!folio_test_hugetlb_mte_tagged(folio)); > else > - WARN_ON_ONCE(!page_mte_tagged(page)); > + WARN_ON_ONCE(!page_mte_tagged(page) && !is_zero_page(page)); > > /* limit access to the end of the page */ > offset = offset_in_page(addr);