On Mon, Mar 10, 2025 at 06:13:58PM +0000, Mark Rutland wrote: > 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. > > If we only care about synchronous and asymmetric modes, that should be > possible, but that won't work in asynchronous mode. In asynchronous mode > the fault will accumulate into TFSR and will be detected later > asynchronously where it cannot be related to its source and fixed up. > > That means that both read_word_at_a_time() and load_unaligned_zeropad() > are dodgy in async mode. load_unaligned_zeropad() has a __mte_enable_tco_async() call to set PSTATE.TCO if in async mode, so that's covered. read_word_at_a_time() is indeed busted and I've had Vincezo's patches for a couple of years already, they just never made it to the list. > Can we somehow hang this off ARCH_HAS_SUBPAGE_FAULTS? We could, though that was mostly for user-space faults while in-kernel we'd only need something similar if KASAN_HW_TAGS. > ... and is there anything else that deliberately makes accesses that > could straddle objects? So far we only came across load_unaligned_zeropad() and read_word_at_a_time(). I'm not aware of anything else. -- Catalin
