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

[Kalra, Ashish]

Hello Tom,

On 4/24/2025 10:29 AM, Tom Lendacky wrote:
> On 4/24/25 09:27, Ashish Kalra wrote:
>> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>>
>> 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@xxxxxxxxxxxxxxx
>> Fixes: 3074152e56c9 ("x86/sev: Convert shared memory back to private on kexec")
>> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
>> ---
>>  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?
>

Yes that makes sense and i will fix it accordingly.

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