Re: [PATCH stable] x86/microcode: do not cache microcode if it will not be used

[Borislav Petkov]

On Mon, Jan 15, 2024 at 11:22:02AM +0100, Paolo Bonzini wrote:
> [ Upstream commit a7939f01672034a58ad3fdbce69bb6c665ce0024 ]
> 
> Builtin/initrd microcode will not be used the ucode loader is disabled.
> But currently, save_microcode_in_initrd is always performed and it
> accesses MSR_IA32_UCODE_REV even if dis_ucode_ldr is true, and in
> particular even if X86_FEATURE_HYPERVISOR is set; the TDX module does not
> implement the MSR and the result is a call trace at boot for TDX guests.
> 
> Mainline Linux fixed this as part of a more complex rework of microcode
> caching that went into 6.7 (see in particular commits dd5e3e3ca6,
> "x86/microcode/intel: Simplify early loading"; and a7939f0167203,
> "x86/microcode/amd: Cache builtin/initrd microcode early").  Do the bare
> minimum in stable kernels, setting initrd_gone just like mainline Linux
> does in mark_initrd_gone().
> 
> Note that save_microcode_in_initrd() is not in the microcode application
> path, which runs with paging disabled on 32-bit systems, so it can (and
> has to) use dis_ucode_ldr instead of check_loader_disabled_ap().

.. simply because save_microcode_in_initrd() is a fs_initcall and by
then 32-bit has managed to enable paging.

> Cc: stable@xxxxxxxxxxxxxxx # v6.6+
> Cc: x86@xxxxxxxxxx # v6.6+
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>  arch/x86/kernel/cpu/microcode/core.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
> index 35d39a13dc90..503b5da56685 100644
> --- a/arch/x86/kernel/cpu/microcode/core.c
> +++ b/arch/x86/kernel/cpu/microcode/core.c
> @@ -214,6 +214,11 @@ static int __init save_microcode_in_initrd(void)
>  	struct cpuinfo_x86 *c = &boot_cpu_data;
>  	int ret = -EINVAL;
>  
> +	if (dis_ucode_ldr) {
> +		ret = 0;
> +		goto out;
> +	}
> +
>  	switch (c->x86_vendor) {
>  	case X86_VENDOR_INTEL:
>  		if (c->x86 >= 6)
> @@ -227,6 +230,7 @@ static int __init save_microcode_in_initrd(void)
>  		break;
>  	}
>  
> +out:
>  	initrd_gone = true;
>  
>  	return ret;
> -- 

Makes sense to me.

Acked-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>

-- 
Regards/Gruss,
    Boris.

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