On Mon, Mar 10, 2025 at 05:37:50PM +0000, Catalin Marinas 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. Agh, right, I keep forgetting that this can read past the end of the actual allocation. I'd agree, load_unaligned_zeropad() makes sense there. > > > > 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 > > 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. Okay, sounds good. (And with a mild thread-merge: yes, folks want to use KASAN_HW_TAGS=y in production.) -- Kees Cook
