linux-kernel - Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access

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Date:   Thu, 2 Apr 2020 10:38:32 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, airlied@...ux.ie,
        daniel@...ll.ch, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and
 user_write_access_begin/end

On Thu, Apr 02, 2020 at 07:03:28PM +0200, Christophe Leroy wrote:
> > What should we do about arm and s390?  There we want a cookie passed
> > from beginning of block to its end; should that be a return value?
> 
> That was the way I implemented it in January, see
> https://patchwork.ozlabs.org/patch/1227926/
> 
> There was some discussion around that and most noticeable was:
> 
> H. Peter (hpa) said about it: "I have *deep* concern with carrying state in
> a "key" variable: it's a direct attack vector for a crowbar attack,
> especially since it is by definition live inside a user access region."

I share this concern -- we want to keep user/kernel access as static as
possible. It should be provable with static analysis, etc (e.g. objtool
does this already for x86).

Since this doesn't disrupt existing R+W access, I'd prefer the design of
this series as-is.

-- 
Kees Cook