[RFC PATCH 01/17] x86/boot: Check boot param's cc_blob_address for direct boot mode

[Tianyu Lan <ltykernel@gmail.com>]

From: Tianyu Lan <tiala@microsoft.com>

Hypervisor may pass cc blob address directly into boot param's cc
blob address in the direct boot mode. Check cc blcb hdr magic first
in the sev_enable() and use it as cc blob address if check successfully.

Signed-off-by: Tianyu Lan <tiala@microsoft.com>
---
 arch/x86/boot/compressed/sev.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index c93930d5ccbd..960968f8bf75 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -272,17 +272,24 @@ static void enforce_vmpl0(void)
 
 void sev_enable(struct boot_params *bp)
 {
+	struct cc_blob_sev_info *cc_info;
 	unsigned int eax, ebx, ecx, edx;
 	struct msr m;
 	bool snp;
 
 	/*
-	 * bp->cc_blob_address should only be set by boot/compressed kernel.
-	 * Initialize it to 0 to ensure that uninitialized values from
-	 * buggy bootloaders aren't propagated.
+	 * bp->cc_blob_address should only be set by boot/compressed
+	 * kernel and hypervisor with direct boot mode. Initialize it
+	 * to 0 after checking in order to ensure that uninitialized
+	 * values from buggy bootloaders aren't propagated.
 	 */
-	if (bp)
-		bp->cc_blob_address = 0;
+	if (bp) {
+		cc_info = (struct cc_blob_sev_info *)(unsigned long)
+			bp->cc_blob_address;
+
+		if (cc_info->magic != CC_BLOB_SEV_HDR_MAGIC)
+			bp->cc_blob_address = 0;
+	}
 
 	/*
 	 * Setup/preliminary detection of SNP. This will be sanity-checked
@@ -374,6 +381,10 @@ static struct cc_blob_sev_info *find_cc_blob(struct boot_params *bp)
 {
 	struct cc_blob_sev_info *cc_info;
 
+	/* Boot kernel would have passed the CC blob via boot_params. */
+	if (bp->cc_blob_address)
+		return (struct cc_blob_sev_info *)(unsigned long)bp->cc_blob_address;
+
 	cc_info = find_cc_blob_efi(bp);
 	if (cc_info)
 		goto found_cc_info;
@@ -416,9 +427,11 @@ bool snp_init(struct boot_params *bp)
 	/*
 	 * Pass run-time kernel a pointer to CC info via boot_params so EFI
 	 * config table doesn't need to be searched again during early startup
-	 * phase.
+	 * phase. Hypervisor also may popualte cc_blob_address directyly
+	 * in direct boot mode.
 	 */
-	bp->cc_blob_address = (u32)(unsigned long)cc_info;
+	if (!bp->cc_blob_address)
+		bp->cc_blob_address = (u32)(unsigned long)cc_info;
 
 	return true;
 }
-- 
2.25.1