linux-kernel - Re: KASAN: use-after-scope in ext4_group_desc

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Message-ID: <20181009132625.GC21519@hc>
Date:   Tue, 9 Oct 2018 13:26:34 +0000
From:   Jan Glauber <Jan.Glauber@...ium.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
CC:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: KASAN: use-after-scope in ext4_group_desc_csum

On Fri, Oct 05, 2018 at 05:32:07PM +0200, Dmitry Vyukov wrote:
[...]

> This all makes me think that somebody else has left these 0xf8 in
> shadow before ext4_map_blocks started executing.
> Unfortunately debugging garbage in stack shadow is not completely
> trivial and there is no common recipe. I don't have setup to run arm64
> kernel at the moment. I would try to locate that garbage in stack
> shadow earlier, e.g. calling another function before ext4_map_blocks,
> implementing that function in mm/kasan/kasan.c (non-instrumented
> itself) and then try to scan stack and verify presence of 0xf8
> garbage. If this works out, then try to catch garbage earlier and/or
> try to figure out what function left that garbage (that's possible by
> locating 0x41b58ab3 magic:
> https://bugzilla.kernel.org/show_bug.cgi?id=198435).

Thanks a lot for your analysis! I'll try to debug this further but as
you pointed out it might be difficult to catch who writes beforehand to
that location.

--Jan