Re: [PATCH] x86/sev: Fix making shared pages private during kdump

[Tom Lendacky <thomas.lendacky@amd.com>]

On 4/24/25 09:27, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@amd.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 2MB page.
> 
> There is a bug in this additional check for GHCB page contained
> within a 2MB page which causes any shared page just below the
> per-cpu GHCB getting skipped from being transitioned back to private
> before kdump preparation which subsequently causes a 0x404 #VC
> exception when this shared page is accessed later while dumping guest
> memory during vmcore generation via kdump. 
> 
> Correct the detection and handling of GHCB pages contained within
> a 2MB page.
> 
> Cc: stable@vger.kernel.org
> Fixes: 3074152e56c9 ("x86/sev: Convert shared memory back to private on kexec")
> Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> ---
>  arch/x86/coco/sev/core.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index 2c27d4b3985c..16d874f4dcd3 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -926,7 +926,13 @@ 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 2MB page containing the GHCB page */

s/2MB page/a huge 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;
>  			}
> @@ -1106,6 +1112,9 @@ void snp_kexec_finish(void)
>  		ghcb = &data->ghcb_page;
>  		pte = lookup_address((unsigned long)ghcb, &level);
>  		size = page_level_size(level);
> +		/* Handle the case of 2MB page containing the GHCB page */
> +		if (level > PG_LEVEL_4K)
> +			ghcb = (struct ghcb *)((unsigned long)ghcb & PMD_MASK);

For safety, shouldn't the mask be based on the level/size that is returned?

Thanks,
Tom

>  		set_pte_enc(pte, level, (void *)ghcb);
>  		snp_set_memory_private((unsigned long)ghcb, (size / PAGE_SIZE));
>  	}