[PATCH] x86/boot: Don't propagate uninitialized boot_params->cc_blob_address

[Michael Roth]

In some cases, bootloaders will leave boot_params->cc_blob_address
uninitialized rather than zeroing it out. This field is only meant to be
set by the boot/compressed kernel in order to pass information to the
uncompressed kernel when SEV-SNP support is enabled.

Therefore, there are no cases where the bootloader-provided values
should be treated as anything other than garbage. Otherwise, the
uncompressed kernel may attempt to access this bogus address, leading to
a crash during early boot.

Normally, sanitize_boot_params() would be used to clear out such fields
but that happens too late: sev_enable() may have already initialized
it to a valid value that should not be zeroed out. Instead, have
sev_enable() zero it out unconditionally beforehand.

Also ensure this happens for !CONFIG_AMD_MEM_ENCRYPT as well by also
including this handling in the sev_enable() stub function.

  [ bp: Massage commit message and comments. ]

Fixes: b190a043c49a ("x86/sev: Add SEV-SNP feature detection/setup")
Reported-by: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>
Reported-by: watnuss@xxxxxx
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216387
Link: https://lore.kernel.org/r/20220823160734.89036-1-michael.roth@xxxxxxx
---
 arch/x86/boot/compressed/misc.h | 12 +++++++++++-
 arch/x86/boot/compressed/sev.c  |  8 ++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 4910bf230d7b..62208ec04ca4 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -132,7 +132,17 @@ void snp_set_page_private(unsigned long paddr);
 void snp_set_page_shared(unsigned long paddr);
 void sev_prep_identity_maps(unsigned long top_level_pgt);
 #else
-static inline void sev_enable(struct boot_params *bp) { }
+static inline void sev_enable(struct boot_params *bp)
+{
+	/*
+	 * bp->cc_blob_address should only be set by boot/compressed kernel.
+	 * Initialize it to 0 unconditionally (thus here in this stub too) to
+	 * ensure that uninitialized values from buggy bootloaders aren't
+	 * propagated.
+	 */
+	if (bp)
+		bp->cc_blob_address = 0;
+}
 static inline void sev_es_shutdown_ghcb(void) { }
 static inline bool sev_es_check_ghcb_fault(unsigned long address)
 {
diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 52f989f6acc2..c93930d5ccbd 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -276,6 +276,14 @@ void sev_enable(struct boot_params *bp)
 	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.
+	 */
+	if (bp)
+		bp->cc_blob_address = 0;
+
 	/*
 	 * Setup/preliminary detection of SNP. This will be sanity-checked
 	 * against CPUID/MSR values later.
-- 
2.35.1

-- 
Regards/Gruss,
    Boris.

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