linux-kernel - Re: [PATCH] arch/x86: reset MXCSR to default in kernel_fpu

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Date:   Tue, 2 Jun 2020 10:03:24 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Borislav Petkov <bp@...en8.de>, Shuah Khan <shuah@...nel.org>
Cc:     Petteri Aimonen <jpa@....mail.kapsi.fi>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/x86: reset MXCSR to default in kernel_fpu_begin()

On Tue, Jun 2, 2020 at 3:56 AM Borislav Petkov <bp@...en8.de> wrote:
>
> Hi,
>
> On Tue, Jun 02, 2020 at 01:29:51PM +0300, Petteri Aimonen wrote:
> > The kernel module is not actually x86-specific, even though it is
> > currently only enabled for x86. amdgpu driver already does kernel mode
> > floating point operations on PPC64 also, and the same module could be
> > used to test the same thing there.
>
> Then make it generic please and put the user portion in, say,
> tools/testing/selftests/fpu/ and we can ask ppc people to test it too.
> People might wanna add more stuff to it in the future, which would be
> good.
>
> > To deterministically trigger the bug, the syscall has to come from the
> > same thread that has modified MXCSR. Going through /usr/sbin/modprobe
> > won't work, and manually doing the necessary syscalls for module loading
> > seems too complicated.
>
> Ok, fair enough. But put that file in debugfs pls.

I think I agree.  While it would be delightful to have general
selftest tooling for kernel modules, we don't have that right now, and
having the test just work with an appropriately configured kernel
would be nice.

How about putting the file you frob in
/sys/kernel/debug/selftest_helpers/something_or_other.  The idea would
be that /sys/kernel/debug/selftest_helpers would be a general place
for kernel helpers needed to make selftests work.

--Andy