On Tue, 2021-07-27 at 20:22 +0100, Catalin Marinas 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. This kasan issue only happened on hardware tag-based kasan mode. Because kasan_disable_current() works for generic and sw tag-based kasan. HW tag-based kasan depends on slub so slab will not hit this issue. I think we can just check if HW tag-based kasan is enabled or not and decide to reset the tag as below. if (kasan_has_integrated_init()) // slub case, hw-tag kasan pointer = *(unsigned long *)kasan_reset_tag((void *)ptr); else pointer = *ptr; // slab Is this better or any other suggestions? Any suggestion is appreciated. Thanks, Kuan-Ying Lee >
