linux-kernel - Re: [PATCH v3] x86/sev: Fix making shared pages private during kdump

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Message-ID: <633b73ac-8983-fe38-dcdc-0b6a08388f5d@amd.com>
Date: Thu, 1 May 2025 08:56:00 -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 v3] x86/sev: Fix making shared pages private during kdump

On 4/30/25 18:17, 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.
> 
> While handling the case of GHCB page contained within a huge page
> any shared page just below the GHCB page gets skipped from being
> transitioned back to private during kdump preparation.

Why this was occurring is because the original check was incorrect. The
check for

 ghcb <= addr + size

can result in skipping a range that should not have been skipped because
the "addr + size" is actually the start of a page/range after the end of
the range being checked. If the ghcb address was equal to addr + size,
then it was mistakenly considered part of the range when it really wasn't.

I think the check could have just been changed to:

  if (addr <= ghcb && ghcb < addr + size) {

The new checks are a bit clearer in showing normal pages vs huge pages,
though, but you can clearly see the "ghcb < addr + size" change to do the
right thing in the huge page case.

While it is likely that a GHCB page hasn't been part of a huge page during
all the testing, the change in snp_kexec_finish() to mask the address is
the proper thing to do. It probably doesn't even need the if check as the
mask can just be applied no matter what.

Thanks,
Tom

> 
> This subsequently causes a 0x404 #VC exception when this skipped
> shared page is accessed later while dumping guest memory during
> vmcore generation via kdump.
> 
> Split the initial check for skipping the GHCB page into the page
> being skipped fully containing the GHCB and GHCB being contained 
> within a huge page. Also ensure that the skipped huge page
> containing the GHCB page is transitioned back to private later
> when changing GHCBs to private at end of kdump preparation.
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> Cc: stable@...r.kernel.org
> Fixes: 3074152e56c9 ("x86/sev: Convert shared memory back to private on kexec")
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> ---
>  arch/x86/coco/sev/core.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index d35fec7b164a..1f53383bd1fa 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -1019,7 +1019,13 @@ 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 (level == PG_LEVEL_4K && addr == ghcb) {
> +				skipped_addr = true;
> +				break;
> +			}
> +			if (level > PG_LEVEL_4K && addr <= ghcb &&
> +			    ghcb < addr + size) {
>  				skipped_addr = true;
>  				break;
>  			}
> @@ -1131,8 +1137,8 @@ static void shutdown_all_aps(void)
>  void snp_kexec_finish(void)
>  {
>  	struct sev_es_runtime_data *data;
> +	unsigned long size, mask;
>  	unsigned int level, cpu;
> -	unsigned long size;
>  	struct ghcb *ghcb;
>  	pte_t *pte;
>  
> @@ -1160,6 +1166,10 @@ void snp_kexec_finish(void)
>  		ghcb = &data->ghcb_page;
>  		pte = lookup_address((unsigned long)ghcb, &level);
>  		size = page_level_size(level);
> +		mask = page_level_mask(level);
> +		/* Handle the case of a huge page containing the GHCB page */
> +		if (level > PG_LEVEL_4K)
> +			ghcb = (struct ghcb *)((unsigned long)ghcb & mask);
>  		set_pte_enc(pte, level, (void *)ghcb);
>  		snp_set_memory_private((unsigned long)ghcb, (size / PAGE_SIZE));
>  	}