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Subject: Re: [PATCH v3] kasan: add memory corruption identification for
 software tag-based mode
To:     Walter Wu <walter-zh.wu@mediatek.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        Miles Chen <miles.chen@mediatek.com>
Cc:     kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, wsd_upstream@mediatek.com
References: <20190613081357.1360-1-walter-zh.wu@mediatek.com>
From:   Andrey Ryabinin <aryabinin@virtuozzo.com>
Message-ID: <da7591c9-660d-d380-d59e-6d70b39eaa6b@virtuozzo.com>
Date:   Thu, 13 Jun 2019 15:27:09 +0300
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On 6/13/19 11:13 AM, Walter Wu wrote:
> This patch adds memory corruption identification at bug report for
> software tag-based mode, the report show whether it is "use-after-free"
> or "out-of-bound" error instead of "invalid-access" error.This will make
> it easier for programmers to see the memory corruption problem.
> 
> Now we extend the quarantine to support both generic and tag-based kasan.
> For tag-based kasan, the quarantine stores only freed object information
> to check if an object is freed recently. When tag-based kasan reports an
> error, we can check if the tagged addr is in the quarantine and make a
> good guess if the object is more like "use-after-free" or "out-of-bound".
> 


We already have all the information and don't need the quarantine to make such guess.
Basically if shadow of the first byte of object has the same tag as tag in pointer than it's out-of-bounds,
otherwise it's use-after-free.

In pseudo-code it's something like this:

u8 object_tag = *(u8 *)kasan_mem_to_shadow(nearest_object(cacche, page, access_addr));

if (access_addr_tag == object_tag && object_tag != KASAN_TAG_INVALID)
	// out-of-bounds
else
	// use-after-free