From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 0A0572356CC; Mon, 10 Mar 2025 19:37:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741635464; cv=none; b=f09Z+QxytirLFmSe4+cCwWXKpQ47PoOBR3aKG833jwyWFeyl9IKb2uIhG7sn3Ullq0tycS9gIvv9NVqoYRoM5aDd+sjsp3ZYUQ6017W+5xcojctji9kucHQLfJiz525TmZs2dX5FXxfYq4CAZKhIRpxlJmUz6hGXZpwnFopHV8Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1741635464; c=relaxed/simple; bh=c1iYrSO8Kd/12THIxsVRKDsAM0Bj4Yao50BlyA3CgBM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Mr6wJ87uhj6dXvwYVxAM8dKmaUlUMmmqt6xllOd0gjQETRNLzzOj8MZkNznn+LBEISL6tcHrXzVIKgyLv0PinomdJA+fsVhBrez7pET/bZ7G7L74cFCyoyg+x6yV/GPaZ1uw5oQeBufRBqSSCa7OLsigDztwGBpcSAa4tARd1QE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com 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 8194A152B; Mon, 10 Mar 2025 12:37:52 -0700 (PDT) Received: from J2N7QTR9R3 (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B96A83F694; Mon, 10 Mar 2025 12:37:38 -0700 (PDT) Date: Mon, 10 Mar 2025 19:37:32 +0000 From: Mark Rutland To: Catalin Marinas Cc: Kees Cook , 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: On Mon, Mar 10, 2025 at 06:40:11PM +0000, Catalin Marinas wrote: > On Mon, Mar 10, 2025 at 06:13:58PM +0000, Mark Rutland wrote: > > On Mon, Mar 10, 2025 at 05:37:50PM +0000, Catalin Marinas wrote: > > > 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. > > > > If we only care about synchronous and asymmetric modes, that should be > > possible, but that won't work in asynchronous mode. In asynchronous mode > > the fault will accumulate into TFSR and will be detected later > > asynchronously where it cannot be related to its source and fixed up. > > > > That means that both read_word_at_a_time() and load_unaligned_zeropad() > > are dodgy in async mode. > > load_unaligned_zeropad() has a __mte_enable_tco_async() call to set > PSTATE.TCO if in async mode, so that's covered. read_word_at_a_time() is > indeed busted and I've had Vincezo's patches for a couple of years > already, they just never made it to the list. Sorry, I missed the __mte_{enable,disable}_tco_async() calls. So long as we're happy to omit the check in that case, that's fine. I was worried that ex_handler_load_unaligned_zeropad() might not do the right thing in response to a tag check fault (e.g. access the wrong 8 bytes), but it looks as though that's ok due to the way it generates the offset and the aligned pointer. If load_unaligned_zeropad() is handed a string that starts with an unexpected tag (and even if that starts off aligned), ex_handler_load_unaligned_zeropad() will access that and cause another tag check fault, which will be reported. Mark.