Re: [PATCH 1/2] kasan, mm: reset tag when access metadata

[Catalin Marinas]

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.

-- 
Catalin