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Message-Id: <20220815134223.740112-5-ardb@kernel.org>
Date: Mon, 15 Aug 2022 15:42:21 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: linux-kernel@...r.kernel.org
Cc: linux-efi@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Michael Roth <michael.roth@....com>
Subject: [PATCH 4/6] x86/compressed: move startup32_check_sev_cbit out of startup code
Move startup32_check_sev_cbit() out of head_64.S and turn it into an
ordinary function using the ordinary calling conventions, rather than
preserving and restoring the registers that are known to be live at the
call site.
Also reorder the call with the EFI mixed mode check, so we are not
omitting the call to startup32_check_sev_cbit() on mixed mode systems
implicitly, even if the set of machines supporting both features is
empty.
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
arch/x86/boot/compressed/head_64.S | 83 ++------------------
arch/x86/boot/compressed/mem_encrypt.S | 65 +++++++++++++++
2 files changed, 70 insertions(+), 78 deletions(-)
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 1ca2ed52f93c..382ed3d8b26a 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -251,6 +251,11 @@ SYM_FUNC_START(startup_32)
movl $__BOOT_TSS, %eax
ltr %ax
+#ifdef CONFIG_AMD_MEM_ENCRYPT
+ /* Check if the C-bit position is correct when SEV is active */
+ call startup32_check_sev_cbit
+#endif
+
/*
* Setup for the jump to 64bit mode
*
@@ -266,9 +271,6 @@ SYM_FUNC_START(startup_32)
leal rva(startup_64_mixedmode)(%ebp), %eax
jne 1f
#endif
- /* Check if the C-bit position is correct when SEV is active */
- call startup32_check_sev_cbit
-
leal rva(startup_64)(%ebp), %eax
1:
pushl $__KERNEL_CS
@@ -703,81 +705,6 @@ SYM_DATA_START(boot_idt)
.endr
SYM_DATA_END_LABEL(boot_idt, SYM_L_GLOBAL, boot_idt_end)
-#ifdef CONFIG_AMD_MEM_ENCRYPT
- __HEAD
- .code32
-#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(startup32_check_sev_cbit)
-#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(startup32_check_sev_cbit)
-
/*
* Stack and heap for uncompression
*/
diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
index 889450d073ea..3cd3db0da49d 100644
--- a/arch/x86/boot/compressed/mem_encrypt.S
+++ b/arch/x86/boot/compressed/mem_encrypt.S
@@ -241,6 +241,71 @@ SYM_FUNC_START(startup32_load_idt)
RET
SYM_FUNC_END(startup32_load_idt)
+/*
+ * 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(startup32_check_sev_cbit)
+ push %ebp
+ push %ebx
+
+ call 0f
+0: pop %ebp
+
+ /* Check for non-zero sev_status */
+ movl (sev_status - 0b)(%ebp), %eax
+ testl %eax, %eax
+ jz 3f
+
+ /*
+ * 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 */
+ leal (sev_check_data - 0b)(%ebp), %ebp
+ movl %eax, 0(%ebp)
+ movl %ebx, 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, 0(%ebp)
+ jne 4f
+ cmpl %ebx, 4(%ebp)
+ jne 4f
+
+ movl %edx, %cr0 /* Restore previous %cr0 */
+
+3: pop %ebx
+ pop %ebp
+ RET
+
+4: /* Check failed - hlt the machine */
+ hlt
+ jmp 4b
+SYM_FUNC_END(startup32_check_sev_cbit)
+
.data
.balign 8
SYM_DATA(sme_me_mask, .quad 0)
--
2.35.1
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