linux-kernel - Re: [PATCH 0/2] drm/radeon: have the callers of set_memory

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Message-ID: <202001080936.A36005F1@keescook>
Date:   Wed, 8 Jan 2020 09:39:27 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Christian König <christian.koenig@....com>
Cc:     Tianlin Li <tli@...italocean.com>,
        kernel-hardening@...ts.openwall.com,
        Alex Deucher <alexander.deucher@....com>, David1.Zhou@....com,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 0/2] drm/radeon: have the callers of set_memory_*() check
 the return value

On Wed, Jan 08, 2020 at 01:56:47PM +0100, Christian König wrote:
> Am 07.01.20 um 20:25 schrieb Tianlin Li:
> > Right now several architectures allow their set_memory_*() family of
> > functions to fail, but callers may not be checking the return values.
> > If set_memory_*() returns with an error, call-site assumptions may be
> > infact wrong to assume that it would either succeed or not succeed at
> > all. Ideally, the failure of set_memory_*() should be passed up the
> > call stack, and callers should examine the failure and deal with it.
> > 
> > Need to fix the callers and add the __must_check attribute. They also
> > may not provide any level of atomicity, in the sense that the memory
> > protections may be left incomplete on failure. This issue likely has a
> > few steps on effects architectures:
> > 1)Have all callers of set_memory_*() helpers check the return value.
> > 2)Add __must_check to all set_memory_*() helpers so that new uses do
> > not ignore the return value.
> > 3)Add atomicity to the calls so that the memory protections aren't left
> > in a partial state.
> > 
> > This series is part of step 1. Make drm/radeon check the return value of
> > set_memory_*().
> 
> I'm a little hesitate merge that. This hardware is >15 years old and nobody
> of the developers have any system left to test this change on.

If that's true it should be removed from the tree. We need to be able to
correctly make these kinds of changes in the kernel.

> Would it be to much of a problem to just add something like: r =
> set_memory_*(); (void)r; /* Intentionally ignored */.

This seems like a bad idea -- we shouldn't be papering over failures
like this when there is logic available to deal with it.

> Apart from that certainly a good idea to add __must_check to the functions.

Agreed!

-Kees

-- 
Kees Cook