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

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Message-ID: <20200602105649.GB11634@zn.tnic>
Date:   Tue, 2 Jun 2020 12:56:49 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Petteri Aimonen <jpa@....mail.kapsi.fi>
Cc:     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()

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.

> The fesetround() and feenableexcept() are the portable ways to modify 
> MXCSR. The test module does cause Precision Exception and Denormal 
> Exception if those exceptions are unmasked.

Ok.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette