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

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));

}