[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 zero'ing it out. This field is only meant to
be set by the boot/compressed kernel to pass information to the
uncompressed kernel when SEV-SNP support is enabled, so 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 zero'd 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.

Fixes: b190a043c49a ("x86/sev: Add SEV-SNP feature detection/setup")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Jeremi Piotrowski <jpiotrowski@xxxxxxxxxxxxxxxxxxx>
Reported-by: watnuss@xxxxxx
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216387
Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
---
 arch/x86/boot/compressed/misc.h | 11 ++++++++++-
 arch/x86/boot/compressed/sev.c  |  8 ++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
index 4910bf230d7b..aa7889751abc 100644
--- a/arch/x86/boot/compressed/misc.h
+++ b/arch/x86/boot/compressed/misc.h
@@ -132,7 +132,16 @@ 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 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.
-- 
2.25.1