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 D82A919CCEC; Wed, 2 Apr 2025 20:10:46 +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=1743624646; cv=none; b=erN32zZHcOcgYPNHV4QAfDEeebNEJrktQ8bV3WtEIWz/Ojngv9VdaWKTUstH4psEzbw/AosDPnjQXuYiR2LgSFPj0yA8EuR0zqsYBtBvpPHBAaOxgDFSzoPIWQbpx4oL0OssjYENaQPJ1DJucXuOwg5prx1lL9infG5IVppAs0c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1743624646; c=relaxed/simple; bh=8zW6/RMnhtv8yr1yURreD6JKFTv60EVXaB39y2/XfEw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=jiiEfW2inKvroxdxCddJLWr29Lo0LDHRHlV/VpmUPSc99FGsVxR3S3OESGvdzMIfMl9rADkP2sZm8vpWsNIrWEgqosKwo+OFS/6YL7mHQQmSvWzU2dji6NDCv7uOek3u6az1qi3BCMInf0+8MHrpSguPUdvtbUvP/dsiMaGbwbc= 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 EC11FC4CEDD; Wed, 2 Apr 2025 20:10:43 +0000 (UTC) Date: Wed, 2 Apr 2025 21:10:41 +0100 From: Catalin Marinas To: Peter Collingbourne Cc: Alexander Viro , Christian Brauner , Jan Kara , Andrew Morton , Kees Cook , Andy Shevchenko , Andrey Konovalov , Mark Rutland , 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 v4 1/2] string: Add load_unaligned_zeropad() code path to sized_strscpy() Message-ID: References: <20250329000338.1031289-1-pcc@google.com> <20250329000338.1031289-2-pcc@google.com> Precedence: bulk X-Mailing-List: linux-fsdevel@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: <20250329000338.1031289-2-pcc@google.com> On Fri, Mar 28, 2025 at 05:03:36PM -0700, Peter Collingbourne wrote: > diff --git a/lib/string.c b/lib/string.c > index eb4486ed40d25..b632c71df1a50 100644 > --- a/lib/string.c > +++ b/lib/string.c > @@ -119,6 +119,7 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) > if (count == 0 || WARN_ON_ONCE(count > INT_MAX)) > return -E2BIG; > > +#ifndef CONFIG_DCACHE_WORD_ACCESS > #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS > /* > * If src is unaligned, don't cross a page boundary, > @@ -133,12 +134,14 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) > /* If src or dest is unaligned, don't do word-at-a-time. */ > if (((long) dest | (long) src) & (sizeof(long) - 1)) > max = 0; > +#endif > #endif > > /* > - * read_word_at_a_time() below may read uninitialized bytes after the > - * trailing zero and use them in comparisons. Disable this optimization > - * under KMSAN to prevent false positive reports. > + * load_unaligned_zeropad() or read_word_at_a_time() below may read > + * uninitialized bytes after the trailing zero and use them in > + * comparisons. Disable this optimization under KMSAN to prevent > + * false positive reports. > */ > if (IS_ENABLED(CONFIG_KMSAN)) > max = 0; > @@ -146,7 +149,11 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count) > while (max >= sizeof(unsigned long)) { > unsigned long c, data; > > +#ifdef CONFIG_DCACHE_WORD_ACCESS > + c = load_unaligned_zeropad(src+res); > +#else > c = read_word_at_a_time(src+res); > +#endif > if (has_zero(c, &data, &constants)) { > data = prep_zero_mask(c, data, &constants); > data = create_zero_mask(data); Kees mentioned the scenario where this crosses the page boundary and we pad the source with zeros. It's probably fine but there are 70+ cases where the strscpy() return value is checked, I only looked at a couple. Could we at least preserve the behaviour with regards to page boundaries and keep the existing 'max' limiting logic? If I read the code correctly, a fall back to reading one byte at a time from an unmapped page would panic. We also get this behaviour if src[0] is reading from an invalid address, though for arm64 the panic would be in ex_handler_load_unaligned_zeropad() when count >= 8. Reading across tag granule (but not across page boundary) and causing a tag check fault would result in padding but we can live with this and only architectures that do MTE-style tag checking would get the new behaviour. What I haven't checked is whether a tag check fault in ex_handler_load_unaligned_zeropad() would confuse the KASAN logic for MTE (it would be a second tag check fault while processing the first). At a quick look, it seems ok but it might be worth checking. -- Catalin