On 4/30/25 18:17, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.ka...@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 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.lenda...@amd.com>
> Cc: sta...@vger.kernel.org
> Fixes: 3074152e56c9 ("x86/sev: Convert shared memory back to private on
> kexec")
> Signed-off-by: Ashish Kalra <ashish.ka...@amd.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));
> }
