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

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Date:   Tue, 2 Jun 2020 12:50:42 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Petteri Aimonen <jpa@....mail.kapsi.fi>,
        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 Jun 2, 2020, at 10:27 AM, Shuah Khan <skhan@...uxfoundation.org> wrote:
> 
> On 6/2/20 11:03 AM, Andy Lutomirski wrote:
>>> 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.
> 
> Let's extend it to do what we want it to do. I will happy to take
> patches. If you have some concrete ideas on what we can add, please
> do a short summary of what is missing. I will find a way to get this
> done.
> 
>> 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.
> 
> Is this a workaround for the lack of selftest tooling for kernel
> modules? In which case, let's us focus on fix selftest tooling.

The goal here is to have a selftest that runs kernel code as part of its operation. That is, the selftest is, logically, starting in userspace:

setup_evil_state();
ret = call_kernel_helper();
check_some_other_stuff();
undo_evil_state();

And the call_kernel_helper() could be moderately specific to the test.

> 
> thanks,
> -- Shuah