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

[Catalin Marinas <catalin.marinas@arm.com>]

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 <pcc@google.com>
> > 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