linux-kernel - Re: [PATCH] x86/msr: Filter MSR writes

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Message-ID: <20201118175048.GK7472@zn.tnic>
Date:   Wed, 18 Nov 2020 18:50:48 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Mathieu Chouquet-Stringer <me@...hieu.digital>
Cc:     Matthew Garrett <mjg59@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        sean.j.christopherson@...el.com, tony.luck@...el.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>, kernel-team@...com
Subject: Re: [PATCH] x86/msr: Filter MSR writes

On Wed, Nov 18, 2020 at 03:04:27PM +0100, Mathieu Chouquet-Stringer wrote:
> TAINT_CPU_OUT_OF_SPEC now means what it says. Historically it was for
> SMP kernel oops on an officially SMP incapable processor but now it also
> covers CPUs whose MSRs have been incorrectly poked at. Update
> documentation and script to reflect that.
> 
> Signed-off-by: Mathieu Chouquet-Stringer <me@...hieu.digital>
> ---
>  Documentation/admin-guide/tainted-kernels.rst | 11 ++++++-----
>  tools/debugging/kernel-chktaint               |  2 +-
>  2 files changed, 7 insertions(+), 6 deletions(-)

Please fix the text in Documentation/admin-guide/sysctl/kernel.rst also.

> diff --git a/Documentation/admin-guide/tainted-kernels.rst b/Documentation/admin-guide/tainted-kernels.rst
> index f718a2eaf1f6..95f432c43ba0 100644
> --- a/Documentation/admin-guide/tainted-kernels.rst
> +++ b/Documentation/admin-guide/tainted-kernels.rst
> @@ -84,7 +84,7 @@ Bit  Log  Number  Reason that got the kernel tainted
>  ===  ===  ======  ========================================================
>    0  G/P       1  proprietary module was loaded
>    1  _/F       2  module was force loaded
> -  2  _/S       4  SMP kernel oops on an officially SMP incapable processor
> +  2  _/S       4  kernel running on out of spec processor
>    3  _/R       8  module was force unloaded
>    4  _/M      16  processor reported a Machine Check Exception (MCE)
>    5  _/B      32  bad page referenced or some unexpected page flags
> @@ -116,10 +116,11 @@ More detailed explanation for tainting
>   1)  ``F`` if any module was force loaded by ``insmod -f``, ``' '`` if all
>       modules were loaded normally.
>  
> - 2)  ``S`` if the oops occurred on an SMP kernel running on hardware that
> -     hasn't been certified as safe to run multiprocessor.
> -     Currently this occurs only on various Athlons that are not
> -     SMP capable.
> + 2)  ``S`` if the kernel is running on any processor that is out of
> +     specifications (writing to MSRs will trigger this behavior).

People might wonder what "out of specifications" means. I'd say
something along the lines of "the CPU has been put into a not supported
configuration, therefore proper execution cannot be guaranteed". Grep
the tree for TAINT_CPU_OUT_OF_SPEC to see when this gets set, might give
you a better idea of what to say.

> +     Historically, it could also be if an oops occured on a kernel running on
> +     hardware that hasn't been certified as safe to run multiprocessor, such
> +     as various Athlons that are not SMP capable.

And here you can expand on the examples by saying that poking at random
MSRs from userspace is one possible way to mis-configure it.

>   3)  ``R`` if a module was force unloaded by ``rmmod -f``, ``' '`` if all
>       modules were unloaded normally.
> diff --git a/tools/debugging/kernel-chktaint b/tools/debugging/kernel-chktaint
> index 2240cb56e6e5..0b9d93e27910 100755
> --- a/tools/debugging/kernel-chktaint
> +++ b/tools/debugging/kernel-chktaint
> @@ -72,7 +72,7 @@ if [ `expr $T % 2` -eq 0 ]; then
>  	addout " "
>  else
>  	addout "S"
> -	echo " * SMP kernel oops on an officially SMP incapable processor (#2)"
> +	echo " * kernel running on out of spec processor (#2)"

Yeah, can you think of a better formulation than "out of spec
processor"?

The CPU is fine, only its current configuration is not.

Thx.

-- 
Regards/Gruss,
    Boris.

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