linux-kernel - Re: [PATCH v5] x86/sev: Fix making shared pages private during kdump

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Message-ID: <71aecbdd-aa6c-6136-7a5d-c8cdc9cafb30@amd.com>
Date: Wed, 7 May 2025 08:58:29 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ashish Kalra <Ashish.Kalra@....com>, tglx@...utronix.de,
 mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org, bp@...en8.de,
 hpa@...or.com
Cc: michael.roth@....com, nikunj@....com, seanjc@...gle.com, ardb@...nel.org,
 stable@...r.kernel.org, linux-kernel@...r.kernel.org,
 kexec@...ts.infradead.org, linux-coco@...ts.linux.dev
Subject: Re: [PATCH v5] x86/sev: Fix making shared pages private during kdump

On 5/6/25 13:35, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> When the shared pages are being made private during kdump preparation
> there are additional checks to handle shared GHCB pages.
> 
> These additional checks include handling the case of GHCB page being
> contained within a huge page.
> 
> The check for handling the case of GHCB contained within a huge
> page incorrectly skips a page just below the GHCB page from being
> transitioned back to private during kdump preparation.
> 
> This skipped page causes a 0x404 #VC exception when it is accessed
> later while dumping guest memory during vmcore generation via kdump.
> 
> Correct the range to be checked for GHCB contained in a huge page.
> Also ensure that the skipped huge page containing the GHCB page is
> transitioned back to private by applying the correct address mask
> later when changing GHCBs to private at end of kdump preparation.
> 
> Fixes: 3074152e56c9 ("x86/sev: Convert shared memory back to private on kexec")
> Cc: stable@...r.kernel.org
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>

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

> ---
>  arch/x86/coco/sev/core.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index d35fec7b164a..30b74e4e4e88 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -1019,7 +1019,8 @@ static void unshare_all_memory(void)
>  			data = per_cpu(runtime_data, cpu);
>  			ghcb = (unsigned long)&data->ghcb_page;
>  
> -			if (addr <= ghcb && ghcb <= addr + size) {
> +			/* Handle the case of a huge page containing the GHCB page */
> +			if (addr <= ghcb && ghcb < addr + size) {
>  				skipped_addr = true;
>  				break;
>  			}
> @@ -1131,8 +1132,8 @@ static void shutdown_all_aps(void)
>  void snp_kexec_finish(void)
>  {
>  	struct sev_es_runtime_data *data;
> +	unsigned long size, addr;
>  	unsigned int level, cpu;
> -	unsigned long size;
>  	struct ghcb *ghcb;
>  	pte_t *pte;
>  
> @@ -1160,8 +1161,10 @@ void snp_kexec_finish(void)
>  		ghcb = &data->ghcb_page;
>  		pte = lookup_address((unsigned long)ghcb, &level);
>  		size = page_level_size(level);
> -		set_pte_enc(pte, level, (void *)ghcb);
> -		snp_set_memory_private((unsigned long)ghcb, (size / PAGE_SIZE));
> +		/* Handle the case of a huge page containing the GHCB page */
> +		addr = (unsigned long)ghcb & page_level_mask(level);
> +		set_pte_enc(pte, level, (void *)addr);
> +		snp_set_memory_private(addr, (size / PAGE_SIZE));
>  	}
>  }
>