On Sun, Apr 4, 2021 at 12:31 AM Marco Elver <elver@xxxxxxxxxx> wrote: > > However, given the above, I think we need to explain this in the > commit message (which also makes the dependency between these 2 > patches clear) and add a comment above the new kasan_unpoison_range(). > That is, if we still think this is the right fix -- I'm not entirely > sure it is. > > Because what I gather from "kasan: initialize shadow to TAG_INVALID > for SW_TAGS", is the requirement that "0xFF pointer tag is a match-all > tag, it doesn't matter what tag the accessed memory has". > > While KFENCE memory is accessible through the slab API, and in this > case ksize() calling kasan_check_byte() leading to a failure, the > kasan_check_byte() call is part of the public KASAN API. Which means > that if some subsystem decides to memblock_alloc() some memory, and > wishes to use kasan_check_byte() on that memory but with an untagged > pointer, will get the same problem as KFENCE: with generic and HW_TAGS > mode everything is fine, but with SW_TAGS mode things break. It makes sense to allow this function to operate on any kind of memory, including memory that hasn't been previously marked by KASAN. > To me this indicates the fix is not with KFENCE, but should be in > mm/kasan/sw_tags.c:kasan_byte_accessible(), which should not load the > shadow when the pointer is untagged. The problem isn't in accessing shadow per se. Looking at kasan_byte_accessible() (in both sw_tags.c and kasan.h), the return statement there seems just wrong and redundant. The KASAN_TAG_KERNEL check should come first: return tag == KASAN_TAG_KERNEL || (shadow_byte != KASAN_TAG_INVALID && tag == shadow_byte); This way, if the pointer tag is KASAN_TAG_KERNEL, the memory is accessible no matter what the memory tag is. But then the KASAN_TAG_INVALID check isn't needed, as this value is never assigned to a pointer tag. Which brings us to: return tag == KASAN_TAG_KERNEL || tag == shadow_byte; Which is essentially the same check that kasan_check_range() performs. Although, kasan_check_range() also checks that the shadow is < KASAN_SHADOW_START. It makes makes sense to add this check into kasan_byte_accessible() as well, before accessing shadow. Thanks!
