RE: [PATCH 1/2] x86, microcode: Sanitize per-cpu microcode reloading interface

["Yu, Fenghua" <fenghua.yu@xxxxxxxxx>]

> From: Borislav Petkov <bp@xxxxxxxxx>
> Date: Tue, 19 Jun 2012 18:03:30 +0200
> Subject: [PATCH 1/2] x86, microcode: Sanitize per-cpu microcode
> reloading interface
> To: X86-ML <x86@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>,
> Thomas Gleixner <tglx@xxxxxxxxxxxxx>, LKML
> <linux-kernel@xxxxxxxxxxxxxxx>, Andreas Herrmann
> <andreas.herrmann3@xxxxxxx>, Borislav Petkov
> <borislav.petkov@xxxxxxx>, Henrique de Moraes Holschuh
> <hmh@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>,
> stable@xxxxxxxxxxxxxxx
> 
> From: Borislav Petkov <borislav.petkov@xxxxxxx>
> 
> Microcode reloading in a per-core manner is a very bad idea for both
> major x86 vendors. And the thing is, we have such interface with which
> we can end up with different microcode versions applied on different
> cores of an otherwise homogeneous wrt (family,model,stepping) system.
> 
> So turn off the possibility of doing that per core and allow it only
> system-wide.
> 
> This is a minimal fix which we'd like to see in stable too thus the
> more-or-less arbitrary decision to allow system-wide reloading only on
> the BSP:
> 
> $ echo 1 > /sys/devices/system/cpu/cpu0/microcode/reload
> ...
> 
> and disable the interface on the other cores:
> 
> $ echo 1 > /sys/devices/system/cpu/cpu23/microcode/reload
> -bash: echo: write error: Invalid argument
> 
> Also, allowing the reload only from one CPU (the BSP in
> that case) doesn't allow the reload procedure to degenerate
> into an O(n^2) deal when triggering reloads from all
> /sys/devices/system/cpu/cpuX/microcode/reload sysfs nodes
> simultaneously.
> 
> A more generic fix will follow.
> 
> Cc: Henrique de Moraes Holschuh <hmh@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Borislav Petkov <borislav.petkov@xxxxxxx>
> ---
>  arch/x86/kernel/microcode_core.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/microcode_core.c
> b/arch/x86/kernel/microcode_core.c
> index fbdfc6917180..24b852b61be3 100644
> --- a/arch/x86/kernel/microcode_core.c
> +++ b/arch/x86/kernel/microcode_core.c
> @@ -298,19 +298,31 @@ static ssize_t reload_store(struct device *dev,
>  			    const char *buf, size_t size)
>  {
>  	unsigned long val;
> -	int cpu = dev->id;
> -	ssize_t ret = 0;
> +	int cpu;
> +	ssize_t ret = 0, tmp_ret;
> +
> +	/* allow reload only from the BSP */
> +	if (boot_cpu_data.cpu_index != dev->id)
> +		return -EINVAL;

With the /sys/devices/system/cpu/microcode/reload interface in your patch 2/2, this will be broken, right? With the new interface, reload_store() can be executed on any cpu or dev. I think you need to remove this check if working with the patch 2/2.

Thanks.

-Fenghua
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