linux-kernel - Re: [PATCH] x86/kexec: Fix bug with call depth tracking

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Date: Mon, 3 Jun 2024 08:30:26 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, David Kaplan <david.kaplan@....com>,
 Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] x86/kexec: Fix bug with call depth tracking

On 6/3/24 03:30, Borislav Petkov wrote:
> From: David Kaplan <david.kaplan@....com>
> 
> The call to cc_platform_has() triggers a fault and system crash if call depth
> tracking is active because the GS segment has been reset by load_segments() and
> GS_BASE is now 0 but call depth tracking uses per-CPU variables to operate.
> 
> Call cc_platform_has() earlier in the function when GS is still valid.
> 
>    [ bp: Massage. ]
> 
> Signed-off-by: David Kaplan <david.kaplan@....com>
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>

Minor nit below, otherwise:

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

Does it need a Fixes: tag for call depth tracking (before the change to 
cc_platform_has() it was a call to sme_active())?

> ---
>   arch/x86/kernel/machine_kexec_64.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index b180d8e497c3..90b1946c06e5 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -295,8 +295,15 @@ void machine_kexec_cleanup(struct kimage *image)
>   void machine_kexec(struct kimage *image)
>   {
>   	unsigned long page_list[PAGES_NR];
> -	void *control_page;
> +        unsigned int host_mem_enc_active;

Looks like spaces used here instead of tabs.

Thanks,
Tom

>   	int save_ftrace_enabled;
> +	void *control_page;
> +
> +	/*
> +	 * This must be done before load_segments() since if call depth tracking
> +	 * is used then GS must be valid to make any function calls.
> +	 */
> +	host_mem_enc_active = cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT);
>   
>   #ifdef CONFIG_KEXEC_JUMP
>   	if (image->preserve_context)
> @@ -358,7 +365,7 @@ void machine_kexec(struct kimage *image)
>   				       (unsigned long)page_list,
>   				       image->start,
>   				       image->preserve_context,
> -				       cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT));
> +				       host_mem_enc_active);
>   
>   #ifdef CONFIG_KEXEC_JUMP
>   	if (image->preserve_context)