linux-kernel - Re: [PATCH] string: Disable read_word_at_a_time() optimizations if kernel MTE is enabled

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Message-ID: <CAMn1gO6Zy1W0M_JsHcX6pcOp8cb7=uWUScf3WGxKTRm=AjXKKA@mail.gmail.com>
Date: Tue, 18 Mar 2025 14:41:39 -0700
From: Peter Collingbourne <pcc@...gle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Kees Cook <kees@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Andrew Morton <akpm@...ux-foundation.org>, Andy Shevchenko <andy@...nel.org>, 
	Andrey Konovalov <andreyknvl@...il.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, stable@...r.kernel.org, 
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] string: Disable read_word_at_a_time() optimizations if
 kernel MTE is enabled

On Mon, Mar 10, 2025 at 10:37 AM Catalin Marinas
<catalin.marinas@....com> 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.

Agreed, strscpy() should be using load_unaligned_zeropad() if
available. We can also disable the code that checks for page
boundaries if load_unaligned_zeropad() is available.

The only other use of read_word_at_a_time() is the one in
dentry_string_cmp(). At the time that I sent the patch I hadn't
noticed the comment in dentry_string_cmp() stating that its argument
to read_word_at_a_time() is aligned. Since calling
read_word_at_a_time() is fine if the pointer is aligned but partially
invalid (not possible with MTE but it is possible with SW KASAN which
uses a 1:1 shadow mapping), I left this user as-is. In other words, I
didn't make read_word_at_a_time() arch-specific as in Vincenzo's
series.

I sent a v2 with my patch to switch strscpy() over to using
load_unaligned_zeropad() if available, as well as the patch adding
tests from Vincenzo's series (which had some issues that I fixed).

Peter

> > > Signed-off-by: Peter Collingbourne <pcc@...gle.com>
> > > Link: https://linux-review.googlesource.com/id/If4b22e43b5a4ca49726b4bf98ada827fdf755548
> > > Fixes: 94ab5b61ee16 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS")
> > > Cc: stable@...r.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