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

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Date:   Tue, 27 Jul 2021 09:10:51 +0200
From:   Marco Elver <elver@...gle.com>
To:     Kuan-Ying Lee <Kuan-Ying.Lee@...iatek.com>
Cc:     Nicholas Tang <nicholas.tang@...iatek.com>,
        Andrew Yang <andrew.yang@...iatek.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 1/2] kasan, mm: reset tag when access metadata

+Cc Catalin

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

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?

Perhaps this should have been "kmemleak, kasan: reset pointer tags to
avoid false positives" ?

> ---
>  mm/kmemleak.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> index 228a2fbe0657..73d46d16d575 100644
> --- a/mm/kmemleak.c
> +++ b/mm/kmemleak.c
> @@ -290,7 +290,7 @@ static void hex_dump_object(struct seq_file *seq,
>         warn_or_seq_printf(seq, "  hex dump (first %zu bytes):\n", len);
>         kasan_disable_current();
>         warn_or_seq_hex_dump(seq, DUMP_PREFIX_NONE, HEX_ROW_SIZE,
> -                            HEX_GROUP_SIZE, ptr, len, HEX_ASCII);
> +                            HEX_GROUP_SIZE, kasan_reset_tag((void *)ptr), len, HEX_ASCII);
>         kasan_enable_current();
>  }
>
> @@ -1171,7 +1171,7 @@ static bool update_checksum(struct kmemleak_object *object)
>
>         kasan_disable_current();
>         kcsan_disable_current();
> -       object->checksum = crc32(0, (void *)object->pointer, object->size);
> +       object->checksum = crc32(0, kasan_reset_tag((void *)object->pointer), object->size);
>         kasan_enable_current();
>         kcsan_enable_current();
>
> @@ -1246,7 +1246,7 @@ static void scan_block(void *_start, void *_end,
>                         break;
>
>                 kasan_disable_current();
> -               pointer = *ptr;
> +               pointer = *(unsigned long *)kasan_reset_tag((void *)ptr);
>                 kasan_enable_current();
>
>                 untagged_ptr = (unsigned long)kasan_reset_tag((void *)pointer);
> --
> 2.18.0
>
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