linux-kernel - Re: [PATCH v2 1/5] uaccess: Add user_read_access_begin/end and user_write_access

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Message-Id: <49YBF35KWQz9sSr@ozlabs.org>
Date:   Fri, 29 May 2020 14:20:23 +1000 (AEST)
From:   Michael Ellerman <patch-notifications@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, airlied@...ux.ie,
        daniel@...ll.ch, torvalds@...ux-foundation.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        keescook@...omium.org, hpa@...or.com
Cc:     linux-arch@...r.kernel.org, linux-mm@...ck.org,
        intel-gfx@...ts.freedesktop.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] uaccess: Add user_read_access_begin/end and user_write_access_begin/end

On Fri, 2020-04-03 at 07:20:50 UTC, Christophe Leroy wrote:
> Some architectures like powerpc64 have the capability to separate
> read access and write access protection.
> For get_user() and copy_from_user(), powerpc64 only open read access.
> For put_user() and copy_to_user(), powerpc64 only open write access.
> But when using unsafe_get_user() or unsafe_put_user(),
> user_access_begin open both read and write.
> 
> Other architectures like powerpc book3s 32 bits only allow write
> access protection. And on this architecture protection is an heavy
> operation as it requires locking/unlocking per segment of 256Mbytes.
> On those architecture it is therefore desirable to do the unlocking
> only for write access. (Note that book3s/32 ranges from very old
> powermac from the 90's with powerpc 601 processor, till modern
> ADSL boxes with PowerQuicc II processors for instance so it
> is still worth considering.)
> 
> In order to avoid any risk based of hacking some variable parameters
> passed to user_access_begin/end that would allow hacking and
> leaving user access open or opening too much, it is preferable to
> use dedicated static functions that can't be overridden.
> 
> Add a user_read_access_begin and user_read_access_end to only open
> read access.
> 
> Add a user_write_access_begin and user_write_access_end to only open
> write access.
> 
> By default, when undefined, those new access helpers default on the
> existing user_access_begin and user_access_end.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> Reviewed-by: Kees Cook <keescook@...omium.org>

Applied to powerpc topic/uaccess, thanks.

https://git.kernel.org/powerpc/c/999a22890cb183b918e4372395d24426a755cef2

cheers