linux-kernel - AMD DC graphics display code enables -mhard-float, -msse, -msse2 without any visible FPU state protection

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Message-ID: <CAG48ez2Sx4ELkM94aD_h_J7K7KBOeuGmvZLKRkg3n_f2WoZ_cg@mail.gmail.com>
Date:   Thu, 2 Apr 2020 04:34:52 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Harry Wentland <harry.wentland@....com>,
        Leo Li <sunpeng.li@....com>, amd-gfx@...ts.freedesktop.org,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "David (ChunMing) Zhou" <David1.Zhou@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: AMD DC graphics display code enables -mhard-float, -msse, -msse2
 without any visible FPU state protection

[x86 folks in CC so that they can chime in on the precise rules for this stuff]

Hi!

I noticed that several makefiles under drivers/gpu/drm/amd/display/dc/
turn on floating-point instructions in the compiler flags
(-mhard-float, -msse and -msse2) in order to make the "float" and
"double" types usable from C code without requiring helper functions.

However, as far as I know, code running in normal kernel context isn't
allowed to use floating-point registers without special protection
using helpers like kernel_fpu_begin() and kernel_fpu_end() (which also
require that the protected code never blocks). If you violate that
rule, that can lead to various issues - among other things, I think
the kernel will clobber userspace FPU register state, and I think the
kernel code can blow up if a context switch happens at the wrong time,
since in-kernel task switches don't preserve FPU state.

Is there some hidden trick I'm missing that makes it okay to use FPU
registers here?

I would try testing this, but unfortunately none of the AMD devices I
have here have the appropriate graphics hardware...