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

[Kees Cook <kees@kernel.org>]

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?

> 
> 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

Would it make sense to sort this out so that KASAN_HW_TAGS can be taken
into account at the Kconfig level instead?

> diff --git a/fs/dcache.c b/fs/dcache.c
> index e3634916ffb93..71f0830ac5e69 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -223,7 +223,7 @@ fs_initcall(init_fs_dcache_sysctls);
>   * Compare 2 name strings, return 0 if they match, otherwise non-zero.
>   * The strings are both count bytes long, and count is non-zero.
>   */
> -#ifdef CONFIG_DCACHE_WORD_ACCESS
> +#if defined(CONFIG_DCACHE_WORD_ACCESS) && !defined(CONFIG_KASAN_HW_TAGS)

Why not also the word_at_a_time use in fs/namei.c and lib/siphash.c?

For reference, here are the DCACHE_WORD_ACCESS places:

arch/arm/Kconfig:58:    select DCACHE_WORD_ACCESS if HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/arm64/Kconfig:137: select DCACHE_WORD_ACCESS
arch/powerpc/Kconfig:192:       select DCACHE_WORD_ACCESS if PPC64 && CPU_LITTLE_ENDIAN
arch/riscv/Kconfig:934: select DCACHE_WORD_ACCESS if MMU
arch/s390/Kconfig:154:  select DCACHE_WORD_ACCESS if !KMSAN
arch/x86/Kconfig:160:   select DCACHE_WORD_ACCESS if !KMSAN
arch/x86/um/Kconfig:12: select DCACHE_WORD_ACCESS

>  
>  #include <asm/word-at-a-time.h>
>  /*
> diff --git a/lib/string.c b/lib/string.c
> index eb4486ed40d25..9a43a3824d0d7 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -119,7 +119,8 @@ ssize_t sized_strscpy(char *dest, const char *src, size_t count)
>  	if (count == 0 || WARN_ON_ONCE(count > INT_MAX))
>  		return -E2BIG;
>  
> -#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && \
> +	!defined(CONFIG_KASAN_HW_TAGS)

There are lots more places checking CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS...
Why only here?

And the Kconfigs since I was comparing these against DCACHE_WORD_ACCESS

arch/arc/Kconfig:352:   select HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/arm/Kconfig:107:   select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
arch/arm64/Kconfig:222: select HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/loongarch/Kconfig:140:     select HAVE_EFFICIENT_UNALIGNED_ACCESS if !ARCH_STRICT_ALIGN
arch/m68k/Kconfig:33:   select HAVE_EFFICIENT_UNALIGNED_ACCESS if !CPU_HAS_NO_UNALIGNED
arch/powerpc/Kconfig:246:       select HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/riscv/Kconfig:935: select HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/s390/Kconfig:197:  select HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/x86/Kconfig:238:   select HAVE_EFFICIENT_UNALIGNED_ACCESS
arch/x86/um/Kconfig:13: select HAVE_EFFICIENT_UNALIGNED_ACCESS

>  	/*
>  	 * If src is unaligned, don't cross a page boundary,
>  	 * since we don't know if the next page is mapped.
> -- 
> 2.49.0.rc0.332.g42c0ae87b1-goog
> 

-Kees

-- 
Kees Cook