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 736301DE89C; Mon, 10 Mar 2025 17:37:55 +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=1741628275; cv=none; b=FL7eztX3ETWKgc+NNm7DWwhVdzbR22B/owME1Jlk4TYYXt8p56UpUG//kYHxoJCF6zsMP9OZ0WWm6ArGgm9kCnoxIP5kJIdpIOUKBSD8yr+T0QBnui95aFtK0cWqljhGZA/0ZEd5pLnuj0PcNJH6K5+M2zAEsFWVa1ZjBGgMGEw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741628275; c=relaxed/simple; bh=RGwv37QAnQo/a2wz5AMKCuLUSQ01Tyu2yE4BPDzoJK8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LabITKIJTb7k3K7elEzWFGuLMl7kuY7ZuD6s9xWJU35Ny6o5rIr8E0MvswCdleqxqHQjU0GQTKjHPENOMPsTsIbZzwuqlU7aRSSouSrJlqGFE4Cjgl3CerW1LTPXSSyIcc95AbIPK0tjgeUPiM8ATZoXzjNo5apS2lmDUQ90MtE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 02D57C4CEE5; Mon, 10 Mar 2025 17:37:52 +0000 (UTC) Date: Mon, 10 Mar 2025 17:37:50 +0000 From: Catalin Marinas To: Kees Cook Cc: Peter Collingbourne , Alexander Viro , Christian Brauner , Jan Kara , Andrew Morton , Andy Shevchenko , Andrey Konovalov , 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: References: <20250308023314.3981455-1-pcc@google.com> <202503071927.1A795821A@keescook> 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: <202503071927.1A795821A@keescook> On Fri, Mar 07, 2025 at 07:36:31PM -0800, Kees Cook wrote: > 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? read_word_at_a_time() is used to read 8 bytes, potentially unaligned and beyond the end of string. The has_zero() function is then used to check where the string ends. For this uses, I think we can go with load_unaligned_zeropad() which handles a potential fault and pads the rest with zeroes. > > 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 DCACHE_WORD_ACCESS requires load_unaligned_zeropad() which handles the faults. For some reason, read_word_at_a_time() doesn't expect to fault and it is only used with HAVE_EFFICIENT_UNALIGNED_ACCESS. I guess arm32 only enabled load_unaligned_zeropad() on hardware that supports efficient unaligned accesses (v6 onwards), hence the dependency. > Would it make sense to sort this out so that KASAN_HW_TAGS can be taken > into account at the Kconfig level instead? I don't think we should play with config options but rather sort out the fault path (load_unaligned_zeropad) or disable MTE temporarily. I'd go with the former as long as read_word_at_a_time() is only used for strings in conjunction with has_zero(). I haven't checked. -- Catalin