x86/compressed: Add helper for validating pages in the decompression stage

Many of the integrity guarantees of SEV-SNP are enforced through the
Reverse Map Table (RMP). Each RMP entry contains the GPA at which a
particular page of DRAM should be mapped. The VMs can request the
hypervisor to add pages in the RMP table via the Page State Change
VMGEXIT defined in the GHCB specification.

Inside each RMP entry is a Validated flag; this flag is automatically
cleared to 0 by the CPU hardware when a new RMP entry is created for a
guest. Each VM page can be either validated or invalidated, as indicated
by the Validated flag in the RMP entry. Memory access to a private page
that is not validated generates a #VC. A VM must use the PVALIDATE
instruction to validate a private page before using it.

To maintain the security guarantee of SEV-SNP guests, when transitioning
pages from private to shared, the guest must invalidate the pages before
asking the hypervisor to change the page state to shared in the RMP table.

After the pages are mapped private in the page table, the guest must
issue a page state change VMGEXIT to mark the pages private in the RMP
table and validate them.

Upon boot, BIOS should have validated the entire system memory.
During the kernel decompression stage, early_setup_ghcb() uses
set_page_decrypted() to make the GHCB page shared (i.e. clear encryption
attribute). And while exiting from the decompression, it calls
set_page_encrypted() to make the page private.

Add snp_set_page_{private,shared}() helpers that are used by
set_page_{decrypted,encrypted}() to change the page state in the RMP
table.

  [ bp: Massage commit message and comments. ]

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220307213356.2797205-16-brijesh.singh@amd.com

1 files changed, 46 insertions, 0 deletions

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index eb421787d50a..5f2c26860df7 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -116,6 +116,52 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 /* Include code for early handlers */
 #include "../../kernel/sev-shared.c"
 
+static inline bool sev_snp_enabled(void)
+{
+	return sev_status & MSR_AMD64_SEV_SNP_ENABLED;
+}
+
+static void __page_state_change(unsigned long paddr, enum psc_op op)
+{
+	u64 val;
+
+	if (!sev_snp_enabled())
+		return;
+
+	/*
+	 * If private -> shared then invalidate the page before requesting the
+	 * state change in the RMP table.
+	 */
+	if (op == SNP_PAGE_STATE_SHARED && pvalidate(paddr, RMP_PG_SIZE_4K, 0))
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);
+
+	/* 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();
+
+	/* Read the response of the VMGEXIT. */
+	val = sev_es_rd_ghcb_msr();
+	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);
+
+	/*
+	 * Now that page state is changed in the RMP table, validate it so that it is
+	 * consistent with the RMP entry.
+	 */
+	if (op == SNP_PAGE_STATE_PRIVATE && pvalidate(paddr, RMP_PG_SIZE_4K, 1))
+		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PVALIDATE);
+}
+
+void snp_set_page_private(unsigned long paddr)
+{
+	__page_state_change(paddr, SNP_PAGE_STATE_PRIVATE);
+}
+
+void snp_set_page_shared(unsigned long paddr)
+{
+	__page_state_change(paddr, SNP_PAGE_STATE_SHARED);
+}
+
 static bool early_setup_ghcb(void)
 {
 	if (set_page_decrypted((unsigned long)&boot_ghcb_page))