On Tue, Jul 27, 2021 at 9:22 PM Catalin Marinas [off-list ref] wrote:

On Tue, Jul 27, 2021 at 04:32:02PM +0800, Kuan-Ying Lee wrote:

quoted

On Tue, 2021-07-27 at 09:10 +0200, Marco Elver wrote:

quoted

+Cc Catalin

On Tue, 27 Jul 2021 at 06:00, Kuan-Ying Lee <
Kuan-Ying.Lee@mediatek.com> wrote:

quoted

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 <redacted>

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.

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