linux-kernel - [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

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Message-ID: <20250410132850.3708703-2-ardb+git@google.com>
Date: Thu, 10 Apr 2025 15:28:51 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-efi@...r.kernel.org
Cc: x86@...nel.org, mingo@...nel.org, linux-kernel@...r.kernel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Tom Lendacky <thomas.lendacky@....com>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, 
	Dionna Amalie Glaze <dionnaglaze@...gle.com>, Kevin Loughlin <kevinloughlin@...gle.com>
Subject: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

From: Ard Biesheuvel <ardb@...nel.org>

Communicating with the hypervisor using the shared GHCB page requires
clearing the C bit in the mapping of that page. When executing in the
context of the EFI boot services, the page tables are owned by the
firmware, and this manipulation is not possible.

So switch to a different API for accepting memory in SEV-SNP guests, one
which is actually supported at the point during boot where the EFI stub
may need to accept memory, but the SEV-SNP init code has not executed
yet.

For simplicity, also switch the memory acceptance carried out by the
decompressor when not booting via EFI - this only involves the
allocation for the decompressed kernel, and is generally only called
after kexec, as normal boot will jump straight into the kernel from the
EFI stub.

Cc: Tom Lendacky <thomas.lendacky@....com>,
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>,
Cc: Dionna Amalie Glaze <dionnaglaze@...gle.com>,
Cc: Kevin Loughlin <kevinloughlin@...gle.com>
Co-developed-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
Changes since v2 [0]:
- avoid two separate acceptance APIs; instead, use MSR based page-by-page
  acceptance for the decompressor as well

[0] https://lore.kernel.org/all/20250404082921.2767593-8-ardb+git@google.com/T/#m38389f607accd0cfa83c41c3bd0d410514b023c6

 arch/x86/boot/compressed/sev.c | 67 +++++---------------
 1 file changed, 15 insertions(+), 52 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index bb55934c1cee..89ba168f4f0f 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -164,10 +164,7 @@ bool sev_snp_enabled(void)
 
 static void __page_state_change(unsigned long paddr, enum psc_op op)
 {
-	u64 val;
-
-	if (!sev_snp_enabled())
-		return;
+	u64 val, msr;
 
 	/*
 	 * If private -> shared then invalidate the page before requesting the
@@ -176,6 +173,9 @@ static void __page_state_change(unsigned long paddr, enum psc_op op)
 	if (op == SNP_PAGE_STATE_SHARED)
 		pvalidate_4k_page(paddr, paddr, false);
 
+	/* Save the current GHCB MSR value */
+	msr = sev_es_rd_ghcb_msr();
+
 	/* Issue VMGEXIT to change the page state in RMP table. */
 	sev_es_wr_ghcb_msr(GHCB_MSR_PSC_REQ_GFN(paddr >> PAGE_SHIFT, op));
 	VMGEXIT();
@@ -185,6 +185,9 @@ static void __page_state_change(unsigned long paddr, enum psc_op op)
 	if ((GHCB_RESP_CODE(val) != GHCB_MSR_PSC_RESP) || GHCB_MSR_PSC_RESP_VAL(val))
 		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PSC);
 
+	/* Restore the GHCB MSR value */
+	sev_es_wr_ghcb_msr(msr);
+
 	/*
 	 * Now that page state is changed in the RMP table, validate it so that it is
 	 * consistent with the RMP entry.
@@ -195,11 +198,17 @@ static void __page_state_change(unsigned long paddr, enum psc_op op)
 
 void snp_set_page_private(unsigned long paddr)
 {
+	if (!sev_snp_enabled())
+		return;
+
 	__page_state_change(paddr, SNP_PAGE_STATE_PRIVATE);
 }
 
 void snp_set_page_shared(unsigned long paddr)
 {
+	if (!sev_snp_enabled())
+		return;
+
 	__page_state_change(paddr, SNP_PAGE_STATE_SHARED);
 }
 
@@ -223,56 +232,10 @@ static bool early_setup_ghcb(void)
 	return true;
 }
 
-static phys_addr_t __snp_accept_memory(struct snp_psc_desc *desc,
-				       phys_addr_t pa, phys_addr_t pa_end)
-{
-	struct psc_hdr *hdr;
-	struct psc_entry *e;
-	unsigned int i;
-
-	hdr = &desc->hdr;
-	memset(hdr, 0, sizeof(*hdr));
-
-	e = desc->entries;
-
-	i = 0;
-	while (pa < pa_end && i < VMGEXIT_PSC_MAX_ENTRY) {
-		hdr->end_entry = i;
-
-		e->gfn = pa >> PAGE_SHIFT;
-		e->operation = SNP_PAGE_STATE_PRIVATE;
-		if (IS_ALIGNED(pa, PMD_SIZE) && (pa_end - pa) >= PMD_SIZE) {
-			e->pagesize = RMP_PG_SIZE_2M;
-			pa += PMD_SIZE;
-		} else {
-			e->pagesize = RMP_PG_SIZE_4K;
-			pa += PAGE_SIZE;
-		}
-
-		e++;
-		i++;
-	}
-
-	if (vmgexit_psc(boot_ghcb, desc))
-		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PSC);
-
-	pvalidate_pages(desc);
-
-	return pa;
-}
-
 void snp_accept_memory(phys_addr_t start, phys_addr_t end)
 {
-	struct snp_psc_desc desc = {};
-	unsigned int i;
-	phys_addr_t pa;
-
-	if (!boot_ghcb && !early_setup_ghcb())
-		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PSC);
-
-	pa = start;
-	while (pa < end)
-		pa = __snp_accept_memory(&desc, pa, end);
+	for (phys_addr_t pa = start; pa < end; pa += PAGE_SIZE)
+		__page_state_change(pa, SNP_PAGE_STATE_PRIVATE);
 }
 
 void sev_es_shutdown_ghcb(void)
-- 
2.49.0.504.g3bcea36a83-goog