linux-kernel - Re: [PATCH v2] proc/vmcore: fix clearing user buffer by properly using clear

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Message-Id: <20211115140444.bca2b88cfdd992760a413442@linux-foundation.org>
Date:   Mon, 15 Nov 2021 14:04:44 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Baoquan He <bhe@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Philipp Rudo <prudo@...hat.com>, kexec@...ts.infradead.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] proc/vmcore: fix clearing user buffer by properly
 using clear_user()

On Fri, 12 Nov 2021 10:27:50 +0100 David Hildenbrand <david@...hat.com> wrote:

> To clear a user buffer we cannot simply use memset, we have to use
> clear_user(). With a virtio-mem device that registers a vmcore_cb and has
> some logically unplugged memory inside an added Linux memory block, I can
> easily trigger a BUG by copying the vmcore via "cp":
> 
> ...
>
> Some x86-64 CPUs have a CPU feature called "Supervisor Mode Access
> Prevention (SMAP)", which is used to detect wrong access from the kernel to
> user buffers like this: SMAP triggers a permissions violation on wrong
> access. In the x86-64 variant of clear_user(), SMAP is properly
> handled via clac()+stac().
> 
> To fix, properly use clear_user() when we're dealing with a user buffer.
> 

I added cc:stable, OK?