On Tue, Jul 27, 2021 at 9:22 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Tue, Jul 27, 2021 at 04:32:02PM +0800, Kuan-Ying Lee wrote: > > On Tue, 2021-07-27 at 09:10 +0200, Marco Elver wrote: > > > +Cc Catalin > > > > > > On Tue, 27 Jul 2021 at 06:00, Kuan-Ying Lee < > > > Kuan-Ying.Lee@xxxxxxxxxxxx> wrote: > > > > > > > > Hardware tag-based KASAN doesn't use compiler instrumentation, we > > > > can not use kasan_disable_current() to ignore tag check. > > > > > > > > Thus, we need to reset tags when accessing metadata. > > > > > > > > Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@xxxxxxxxxxxx> > > > > > > This looks reasonable, but the patch title is not saying this is > > > kmemleak, nor does the description say what the problem is. What > > > problem did you encounter? Was it a false positive? > > > > kmemleak would scan kernel memory to check memory leak. > > When it scans on the invalid slab and dereference, the issue > > will occur like below. > > > > So I think we should reset the tag before scanning. > > > > # echo scan > /sys/kernel/debug/kmemleak > > [ 151.905804] > > ================================================================== > > [ 151.907120] BUG: KASAN: out-of-bounds in scan_block+0x58/0x170 > > [ 151.908773] Read at addr f7ff0000c0074eb0 by task kmemleak/138 > > [ 151.909656] Pointer tag: [f7], memory tag: [fe] > > It would be interesting to find out why the tag doesn't match. Kmemleak > should in principle only scan valid objects that have been allocated and > the pointer can be safely dereferenced. 0xfe is KASAN_TAG_INVALID, so it > either goes past the size of the object (into the red zone) or it still > accesses the object after it was marked as freed but before being > released from kmemleak. > > With slab, looking at __cache_free(), it calls kasan_slab_free() before > ___cache_free() -> kmemleak_free_recursive(), so the second scenario is > possible. With slub, however, slab_free_hook() first releases the object > from kmemleak before poisoning it. Based on the stack dump, you are > using slub, so it may be that kmemleak goes into the object red zones. > > I'd like this clarified before blindly resetting the tag. AFAIK, kmemleak scans the whole object including the leftover redzone for kmalloc-allocated objects. Looking at the report, there are 11 0xf7 granules, which amounts to 176 bytes, and the object is allocated from the kmalloc-256 cache. So when kmemleak accesses the last 256-176 bytes, it causes faults, as those are marked with KASAN_KMALLOC_REDZONE == KASAN_TAG_INVALID == 0xfe. Generally, resetting tags in kasan_disable/enable_current() section should be fine to suppress MTE faults, provided those sections had been added correctly in the first place.
