[MODERATED] Re: [patch V4 04/11] x86/speculation/mds: Add mds_clear_cpu_buffer()

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Feb 22, 2019 at 11:24:22PM +0100, speck for Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> The Microarchitectural Data Sampling (MDS) vulernabilities are mitigated by
> clearing the affected CPU buffers. The mechanism for clearing the buffers
> uses the unused and obsolete VERW instruction in combination with a
> microcode update which triggers a CPU buffer clear when VERW is executed.
> 
> Provide a inline function with the assembly magic. The argument of the VERW
> instruction must be a memory operand as documented:
> 
>   "MD_CLEAR enumerates that the memory-operand variant of VERW (for
>    example, VERW m16) has been extended to also overwrite buffers affected
>    by MDS. This buffer overwriting functionality is not guaranteed for the
>    register operand variant of VERW."
> 
> Documentation also recommends to use a writable data segment selector:
> 
>   "The buffer overwriting occurs regardless of the result of the VERW
>    permission check, as well as when the selector is null or causes a
>    descriptor load segment violation. However, for lowest latency we
>    recommend using a selector that indicates a valid writable data
>    segment."
> 
> Add x86 specific documentation about MDS and the internal workings of the
> mitigation.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Reviewed-by: Borislav Petkov <bp@suse.de>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>