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owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Oct 24, 2025 at 03:56:29AM +0200, Andrey Konovalov wrote: > On Fri, Oct 24, 2025 at 3:19 AM Andrey Konovalov wrote: > > > > On Fri, Oct 24, 2025 at 2:41 AM Harry Yoo wrote: > > > > > > Adding more details on how I discovered this and why I care: > > > > > > I was developing a feature that uses unused bytes in s->size as the > > > slabobj_ext metadata. Unlike other metadata where slab disables KASAN > > > when accessing it, this should be unpoisoned to avoid adding complexity > > > and overhead when accessing it. > > > > Generally, unpoisoining parts of slabs that should not be accessed by > > non-slab code is undesirable - this would prevent KASAN from detecting > > OOB accesses into that memory. > > > > An alternative to unpoisoning or disabling KASAN could be to add > > helper functions annotated with __no_sanitize_address that do the > > required accesses. And make them inlined when KASAN is disabled to > > avoid the performance hit. > > > > On a side note, you might also need to check whether SW_TAGS KASAN and > > KMSAN would be unhappy with your changes: > > > > - When we do kasan_disable_current() or metadata_access_enable(), we > > also do kasan_reset_tag(); > > - In metadata_access_enable(), we disable KMSAN as well. > > > > > This warning is from kasan_unpoison(): > > > if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK)) > > > return; > > > > > > on x86_64, the address passed to kasan_{poison,unpoison}() should be at > > > least aligned with 8 bytes. > > > > > > After manual investigation it turns out when the SLAB_STORE_USER flag is > > > specified, any metadata after the original kmalloc request size is > > > misaligned. > > > > > > Questions: > > > - Could it cause any issues other than the one described above? > > > - Does KASAN even support architectures that have issues with unaligned > > > accesses? > > > > Unaligned accesses are handled just fine. It's just that the start of > > any unpoisoned/accessible memory region must be aligned to 8 (or 16 > > for SW_TAGS) bytes due to how KASAN encodes shadow memory values. > > Misread your question: my response was about whether unaligned > accesses are instrumented/checked correctly on architectures that do > support them. Haha, I was a bit confused while reading the reply, turns out we were talking about different things. And yes, I was asking about the case where the architecture doesn't support it. > For architectures that do not: there might indeed be an issue. > Though there's KASAN support for xtensa and I suppose it works > (does xtensa support unaligned accesses?). Looks like 64-bit architectures without HAVE_EFFICIENT_UNALIGNED_ACCESS are assumed to require 64 bit accesses to be 64 bit aligned [1]? [1] https://lore.kernel.org/all/20201214112629.3cf6f240@gandalf.local.home But yeah, the combination of (architectures that do not support unaligned accesses) x (enabling KASAN) x (enabling slab_debug=U) should be pretty rare... ;) > > > - How come we haven't seen any issues regarding this so far? :/ > > > > As you pointed out, we don't unpoison the memory that stores KASAN > > metadata and instead just disable KASAN error reporting. This is done > > deliberately to allow KASAN catching accesses into that memory that > > happen outside of the slab/KASAN code. -- Cheers, Harry / Hyeonggon