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From:   Brijesh Singh <brijesh.singh@amd.com>
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Subject: [PATCH v11 20/45] x86/kernel: Validate ROM memory before accessing when SEV-SNP is active
Date:   Thu, 24 Feb 2022 10:56:00 -0600
Message-ID: <20220224165625.2175020-21-brijesh.singh@amd.com>
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probe_roms() accesses the memory range (0xc0000 - 0x10000) to probe
various ROMs. The memory range is not part of the E820 system RAM
range. The memory range is mapped as private (i.e encrypted) in page
table.

When SEV-SNP is active, all the private memory must be validated before
the access. The ROM range was not part of E820 map, so the guest BIOS
did not validate it. An access to invalidated memory will cause a VC
exception. The guest does not support handling not-validated VC exception
yet, so validate the ROM memory regions before it is accessed.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
---
 arch/x86/kernel/probe_roms.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/probe_roms.c b/arch/x86/kernel/probe_roms.c
index 36e84d904260..319fef37d9dc 100644
--- a/arch/x86/kernel/probe_roms.c
+++ b/arch/x86/kernel/probe_roms.c
@@ -21,6 +21,7 @@
 #include <asm/sections.h>
 #include <asm/io.h>
 #include <asm/setup_arch.h>
+#include <asm/sev.h>
 
 static struct resource system_rom_resource = {
 	.name	= "System ROM",
@@ -197,11 +198,21 @@ static int __init romchecksum(const unsigned char *rom, unsigned long length)
 
 void __init probe_roms(void)
 {
-	const unsigned char *rom;
 	unsigned long start, length, upper;
+	const unsigned char *rom;
 	unsigned char c;
 	int i;
 
+	/*
+	 * The ROM memory range is not part of the e820 table and is therefore not
+	 * pre-validated by BIOS. The kernel page table maps the ROM region as encrypted
+	 * memory, and SNP requires encrypted memory to be validated before access.
+	 * Do that here.
+	 */
+	snp_prep_memory(video_rom_resource.start,
+			((system_rom_resource.end + 1) - video_rom_resource.start),
+			SNP_PAGE_STATE_PRIVATE);
+
 	/* video rom */
 	upper = adapter_rom_resources[0].start;
 	for (start = video_rom_resource.start; start < upper; start += 2048) {
-- 
2.25.1