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@xxxxxxxxxx> > Link: https://linux-review.googlesource.com/id/If4b22e43b5a4ca49726b4bf98ada827fdf755548 > Fixes: 94ab5b61ee16 ("kasan, arm64: enable CONFIG_KASAN_HW_TAGS") > Cc: stable@xxxxxxxxxxxxxxx > --- > 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
