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Subject[PATCH 6/7] x86/boot/compressed/64: Check SEV encryption in 32-bit boot-path
Date
From: Joerg Roedel <jroedel@suse.de>

Check whether the hypervisor reported the correct C-bit when running as
an SEV guest. Using a wrong C-bit position could be used to leak
sensitive data from the guest to the hypervisor.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/boot/compressed/head_64.S | 80 ++++++++++++++++++++++++++
arch/x86/boot/compressed/mem_encrypt.S | 1 +
2 files changed, 81 insertions(+)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index eadaa0a082b8..047af1cba041 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -185,11 +185,18 @@ SYM_FUNC_START(startup_32)
*/
call get_sev_encryption_bit
xorl %edx, %edx
+#ifdef CONFIG_AMD_MEM_ENCRYPT
testl %eax, %eax
jz 1f
subl $32, %eax /* Encryption bit is always above bit 31 */
bts %eax, %edx /* Set encryption mask for page tables */
+ /*
+ * Store the sme_me_mask as an indicator that SEV is active. It will be
+ * set again in startup_64().
+ */
+ movl %edx, rva(sme_me_mask+4)(%ebp)
1:
+#endif

/* Initialize Page tables to 0 */
leal rva(pgtable)(%ebx), %edi
@@ -274,6 +281,9 @@ SYM_FUNC_START(startup_32)
movl %esi, %edx
1:
#endif
+ /* Check if the C-bit position is correct when SEV is active */
+ call sev_startup32_cbit_check
+
pushl $__KERNEL_CS
pushl %eax

@@ -870,6 +880,76 @@ SYM_FUNC_START(startup32_load_idt)
ret
SYM_FUNC_END(startup32_load_idt)
#endif
+
+/*
+ * Check for the correct C-bit position when the startup_32 boot-path is used.
+ *
+ * The check makes use of the fact that all memory is encrypted when paging is
+ * disabled. The function creates 64 bits of random data using the RDRAND
+ * instruction. RDRAND is mandatory for SEV guests, so always available. If the
+ * hypervisor violates that the kernel will crash right here.
+ *
+ * The 64 bits of random data are stored to a memory location and at the same
+ * time kept in the %eax and %ebx registers. Since encryption is always active
+ * when paging is off the random data will be stored encrypted in main memory.
+ *
+ * Then paging is enabled. When the C-bit position is correct all memory is
+ * still mapped encrypted and comparing the register values with memory will
+ * succeed. An incorrect C-bit position will map all memory unencrypted, so that
+ * the compare will use the encrypted random data and fail.
+ */
+SYM_FUNC_START(sev_startup32_cbit_check)
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+ pushl %eax
+ pushl %ebx
+ pushl %ecx
+ pushl %edx
+
+ /* Check for non-zero sev_status */
+ movl rva(sev_status)(%ebp), %eax
+ testl %eax, %eax
+ jz 4f
+
+ /*
+ * Get two 32-bit random values - Don't bail out if RDRAND fails
+ * because it is better to prevent forward progress if no random value
+ * can be gathered.
+ */
+1: rdrand %eax
+ jnc 1b
+2: rdrand %ebx
+ jnc 2b
+
+ /* Store to memory and keep it in the registers */
+ movl %eax, rva(sev_check_data)(%ebp)
+ movl %ebx, rva(sev_check_data+4)(%ebp)
+
+ /* Enable paging to see if encryption is active */
+ movl %cr0, %edx /* Backup %cr0 in %edx */
+ movl $(X86_CR0_PG | X86_CR0_PE), %ecx /* Enable Paging and Protected mode */
+ movl %ecx, %cr0
+
+ cmpl %eax, rva(sev_check_data)(%ebp)
+ jne 3f
+ cmpl %ebx, rva(sev_check_data+4)(%ebp)
+ jne 3f
+
+ movl %edx, %cr0 /* Restore previous %cr0 */
+
+ jmp 4f
+
+3: /* Check failed - hlt the machine */
+ hlt
+ jmp 3b
+
+4:
+ popl %edx
+ popl %ecx
+ popl %ebx
+ popl %eax
+#endif
+ ret
+SYM_FUNC_END(sev_startup32_cbit_check)
/*
* Stack and heap for uncompression
*/
diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index 091502cde070..b80fed167903 100644
--- a/arch/x86/boot/compressed/mem_encrypt.S
+++ b/arch/x86/boot/compressed/mem_encrypt.S
@@ -7,6 +7,7 @@
* Author: Tom Lendacky <thomas.lendacky@amd.com>
*/

+#define rva(X) ((X) - startup_32)
#include <linux/linkage.h>

#include <asm/processor-flags.h>
--
2.30.0
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