linux-kernel - Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}

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Message-ID: <20171028084729.GL20805@n2100.armlinux.org.uk>
Date:   Sat, 28 Oct 2017 09:47:30 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Will Deacon <will.deacon@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, Laura Abbott <labbott@...hat.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks

On Fri, Oct 27, 2017 at 04:41:13PM +0100, Will Deacon wrote:
> Probably a stupid question, but why not just move the access_ok check
> into __{get,put}_user and remove it from {get,put}_user? We can also
> then move the uaccess_{enable,disable}_not_uao calls out from the __
> variants so that we can implement user_access_{begin,end}.

The intent of __{get,put}_user() is to have a fast accessor compared
to {get,put}_user() which does all the full checks.

However, with the uaccess stuff we have now by default, I don't think
it makes much sense - maybe we're better off using copy_{to,from}_user()
in those code paths and fixing up the struct in kernel space rather than
__{get,put}_user()?

I suspect that if we do have the full checks in __{get,put}_user() that
makes the case stronger for doing that - and maybe killing the __
accessors entirely.

Take a look at kernel/signal.c to see a typical usage of the __
accessors.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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