From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2CE23839F4; Sat, 8 Mar 2025 03:36:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741404995; cv=none; b=eMx5GlZ0xkC1qYGFN74SG+iPQe9KPHkUjJfz2reIzgqpRYpr6LyoHHM0Jhkw1qDmiCmEHKNs9XmfiKyUTANRpA5QjNhdPnSn+KCKyz4R7OFAkV0GSllISuYvipQFw2rlxQ/SNF2IUwPeLIikKNxUuaTYZ5QnR+fyDDBecD0HnGA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741404995; c=relaxed/simple; bh=wLEqT0IC9gJ1SaLEGeSi2Lgq48p3Sz8pFixWAnU6Gj4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=DsfwfunSxMxvV3cgkTeX0HCMXhA5IRDhBbRWNGd8XrmN1bJNWLX+ok4TSDF8lfbcqOCwfDKHpaFAfgF44whSCuNOtApVriU6DIJzA+v7UtFOFogiFOHt5IdbuAvQ/YM9k/o1pqLuCEuESxiXFS/dI4Z7ATwYwLs2Rq76GqV0WaE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=H2gEW4Qe; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="H2gEW4Qe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 824E2C4CED1; Sat, 8 Mar 2025 03:36:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1741404994; bh=wLEqT0IC9gJ1SaLEGeSi2Lgq48p3Sz8pFixWAnU6Gj4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H2gEW4QeX38oiBqCP1qkGaua8cdgDgvAnVgZrCCL8gdBNvYqvK+u0Ru4MnaP0B3nU nG4UJK+irKKH3fr8JqFNeIfavwXOXHEez7xR8gfQ2EywxIyKmiGDI9p69+auzLW308 94lClOOtc+06LFdXpuCtGEGl86LiIxTYNAXZ721T8t71QSU29lB1h/AS0TizRnpXdr PvWAU4zREeLaQjzNf9VI2Ovfghe70nqWP570vRzDTHGwJhHJEJpJUsy5zOPmKB0e5i zaTefESg7PDwrAk2G3CMdKBDPFAM6BP6njtX4mI9st8gARwjmqHIPxuxxNiT1m5KOt hhKJZymnSu8LA== Date: Fri, 7 Mar 2025 19:36:31 -0800 From: Kees Cook To: Peter Collingbourne Cc: Alexander Viro , Christian Brauner , Jan Kara , Andrew Morton , Andy Shevchenko , Andrey Konovalov , Catalin Marinas , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org, linux-arm-kernel@lists.infradead.org, stable@vger.kernel.org Subject: Re: [PATCH] string: Disable read_word_at_a_time() optimizations if kernel MTE is enabled Message-ID: <202503071927.1A795821A@keescook> References: <20250308023314.3981455-1-pcc@google.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250308023314.3981455-1-pcc@google.com> On Fri, Mar 07, 2025 at 06:33:13PM -0800, Peter Collingbourne wrote: > The optimized strscpy() and dentry_string_cmp() routines will read 8 > unaligned bytes at a time via the function read_word_at_a_time(), but > this is incompatible with MTE which will fault on a partially invalid > read. The attributes on read_word_at_a_time() that disable KASAN are > invisible to the CPU so they have no effect on MTE. Let's fix the > bug for now by disabling the optimizations if the kernel is built > with HW tag-based KASAN and consider improvements for followup changes. Why is faulting on a partially invalid read a problem? It's still invalid, so ... it should fault, yes? What am I missing? > > Signed-off-by: Peter Collingbourne > Link: https://linux-review.googlesource.com/id/If4b22e43b5a4ca49726b4bf98ada827fdf755548 > Fixes: 94ab5b61ee16 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS") > Cc: stable@vger.kernel.org > --- > fs/dcache.c | 2 +- > lib/string.c | 3 ++- > 2 files changed, 3 insertions(+), 2 deletions(-) Why are DCACHE_WORD_ACCESS and HAVE_EFFICIENT_UNALIGNED_ACCESS separate things? I can see at least one place where it's directly tied: arch/arm/Kconfig:58: select DCACHE_WORD_ACCESS if HAVE_EFFICIENT_UNALIGNED_ACCESS Would it make sense to sort this out so that KASAN_HW_TAGS can be taken into account at the Kconfig level instead? > diff --git a/fs/dcache.c b/fs/dcache.c > index e3634916ffb93..71f0830ac5e69 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -223,7 +223,7 @@ fs_initcall(init_fs_dcache_sysctls); > * Compare 2 name strings, return 0 if they match, otherwise non-zero. > * The strings are both count bytes long, and count is non-zero. > */ > -#ifdef CONFIG_DCACHE_WORD_ACCESS > +#if defined(CONFIG_DCACHE_WORD_ACCESS) && !defined(CONFIG_KASAN_HW_TAGS) Why not also the word_at_a_time use in fs/namei.c and lib/siphash.c? For reference, here are the DCACHE_WORD_ACCESS places: arch/arm/Kconfig:58: select DCACHE_WORD_ACCESS if HAVE_EFFICIENT_UNALIGNED_ACCESS arch/arm64/Kconfig:137: select DCACHE_WORD_ACCESS arch/powerpc/Kconfig:192: select DCACHE_WORD_ACCESS if PPC64 && CPU_LITTLE_ENDIAN arch/riscv/Kconfig:934: select DCACHE_WORD_ACCESS if MMU arch/s390/Kconfig:154: select DCACHE_WORD_ACCESS if !KMSAN arch/x86/Kconfig:160: select DCACHE_WORD_ACCESS if !KMSAN arch/x86/um/Kconfig:12: select DCACHE_WORD_ACCESS > > #include > /* > diff --git a/lib/string.c b/lib/string.c > index eb4486ed40d25..9a43a3824d0d7 100644 > --- a/lib/string.c > +++ b/lib/string.c > @@ -119,7 +119,8 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) > if (count == 0 || WARN_ON_ONCE(count > INT_MAX)) > return -E2BIG; > > -#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS > +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && \ > + !defined(CONFIG_KASAN_HW_TAGS) There are lots more places checking CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS... Why only here? And the Kconfigs since I was comparing these against DCACHE_WORD_ACCESS arch/arc/Kconfig:352: select HAVE_EFFICIENT_UNALIGNED_ACCESS arch/arm/Kconfig:107: select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU arch/arm64/Kconfig:222: select HAVE_EFFICIENT_UNALIGNED_ACCESS arch/loongarch/Kconfig:140: select HAVE_EFFICIENT_UNALIGNED_ACCESS if !ARCH_STRICT_ALIGN arch/m68k/Kconfig:33: select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED arch/powerpc/Kconfig:246: select HAVE_EFFICIENT_UNALIGNED_ACCESS arch/riscv/Kconfig:935: select HAVE_EFFICIENT_UNALIGNED_ACCESS arch/s390/Kconfig:197: select HAVE_EFFICIENT_UNALIGNED_ACCESS arch/x86/Kconfig:238: select HAVE_EFFICIENT_UNALIGNED_ACCESS arch/x86/um/Kconfig:13: select HAVE_EFFICIENT_UNALIGNED_ACCESS > /* > * If src is unaligned, don't cross a page boundary, > * since we don't know if the next page is mapped. > -- > 2.49.0.rc0.332.g42c0ae87b1-goog > -Kees -- Kees Cook