[PATCHv2 18/29] x86/boot: Avoid #VE during boot for TDX platforms

From: Kirill A. Shutemov
Date: Mon Jan 24 2022 - 10:03:24 EST

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From: Sean Christopherson <seanjc@xxxxxxxxxx>

There are a few MSRs and control register bits that the kernel
normally needs to modify during boot. But, TDX disallows
modification of these registers to help provide consistent security
guarantees. Fortunately, TDX ensures that these are all in the correct
state before the kernel loads, which means the kernel does not need to
modify them.

The conditions to avoid are:

* Any writes to the EFER MSR
* Clearing CR0.NE
* Clearing CR3.MCE

This theoretically makes the guest boot more fragile. If, for instance,
EFER was set up incorrectly and a WRMSR was performed, it will trigger
early exception panic or a triple fault, if it's before early
exceptions are set up. However, this is likely to trip up the guest
BIOS long before control reaches the kernel. In any case, these kinds
of problems are unlikely to occur in production environments, and
developers have good debug tools to fix them quickly.

Change the common boot code to work on TDX and non-TDX systems.
This should have no functional effect on non-TDX systems.

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
arch/x86/Kconfig | 1 +
arch/x86/boot/compressed/head_64.S | 25 +++++++++++++++++++++----
arch/x86/boot/compressed/pgtable.h | 2 +-
arch/x86/kernel/head_64.S | 24 ++++++++++++++++++++++--
arch/x86/realmode/rm/trampoline_64.S | 27 +++++++++++++++++++++++----
5 files changed, 68 insertions(+), 11 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1491f25c844e..1c59e02792e4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -885,6 +885,7 @@ config INTEL_TDX_GUEST
depends on X86_64 && CPU_SUP_INTEL
depends on X86_X2APIC
select ARCH_HAS_CC_PLATFORM
+ select X86_MCE
help
Support running as a guest under Intel TDX. Without this support,
the guest kernel can not boot or run under TDX.
diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index fd9441f40457..b576d23d37cb 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -643,12 +643,25 @@ SYM_CODE_START(trampoline_32bit_src)
movl $MSR_EFER, %ecx
rdmsr
btsl $_EFER_LME, %eax
+ /* Avoid writing EFER if no change was made (for TDX guest) */
+ jc 1f
wrmsr
- popl %edx
+1: popl %edx
popl %ecx

/* Enable PAE and LA57 (if required) paging modes */
- movl $X86_CR4_PAE, %eax
+ movl %cr4, %eax
+
+#ifdef CONFIG_X86_MCE
+ /*
+ * Preserve CR4.MCE if the kernel will enable #MC support. Clearing
+ * MCE may fault in some environments (that also force #MC support).
+ * Any machine check that occurs before #MC support is fully configured
+ * will crash the system regardless of the CR4.MCE value set here.
+ */
+ andl $X86_CR4_MCE, %eax
+#endif
+ orl $X86_CR4_PAE, %eax
testl %edx, %edx
jz 1f
orl $X86_CR4_LA57, %eax
@@ -662,8 +675,12 @@ SYM_CODE_START(trampoline_32bit_src)
pushl $__KERNEL_CS
pushl %eax

- /* Enable paging again */
- movl $(X86_CR0_PG | X86_CR0_PE), %eax
+ /*
+ * Enable paging again. Keep CR0.NE set, FERR# is no longer used
+ * to handle x87 FPU errors and clearing NE may fault in some
+ * environments.
+ */
+ movl $(X86_CR0_PG | X86_CR0_NE | X86_CR0_PE), %eax
movl %eax, %cr0

lret
diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
index 6ff7e81b5628..cc9b2529a086 100644
--- a/arch/x86/boot/compressed/pgtable.h
+++ b/arch/x86/boot/compressed/pgtable.h
@@ -6,7 +6,7 @@
#define TRAMPOLINE_32BIT_PGTABLE_OFFSET 0

#define TRAMPOLINE_32BIT_CODE_OFFSET PAGE_SIZE
-#define TRAMPOLINE_32BIT_CODE_SIZE 0x70
+#define TRAMPOLINE_32BIT_CODE_SIZE 0x80

#define TRAMPOLINE_32BIT_STACK_END TRAMPOLINE_32BIT_SIZE

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 9c63fc5988cd..652845cc527e 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -141,7 +141,17 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
1:

/* Enable PAE mode, PGE and LA57 */
- movl $(X86_CR4_PAE | X86_CR4_PGE), %ecx
+ movq %cr4, %rcx
+#ifdef CONFIG_X86_MCE
+ /*
+ * Preserve CR4.MCE if the kernel will enable #MC support. Clearing
+ * MCE may fault in some environments (that also force #MC support).
+ * Any machine check that occurs before #MC support is fully configured
+ * will crash the system regardless of the CR4.MCE value set here.
+ */
+ andl $X86_CR4_MCE, %ecx
+#endif
+ orl $(X86_CR4_PAE | X86_CR4_PGE), %ecx
#ifdef CONFIG_X86_5LEVEL
testl $1, __pgtable_l5_enabled(%rip)
jz 1f
@@ -246,13 +256,23 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
/* Setup EFER (Extended Feature Enable Register) */
movl $MSR_EFER, %ecx
rdmsr
+ /*
+ * Preserve current value of EFER for comparison and to skip
+ * EFER writes if no change was made (for TDX guest)
+ */
+ movl %eax, %edx
btsl $_EFER_SCE, %eax /* Enable System Call */
btl $20,%edi /* No Execute supported? */
jnc 1f
btsl $_EFER_NX, %eax
btsq $_PAGE_BIT_NX,early_pmd_flags(%rip)
-1: wrmsr /* Make changes effective */

+ /* Avoid writing EFER if no change was made (for TDX guest) */
+1: cmpl %edx, %eax
+ je 1f
+ xor %edx, %edx
+ wrmsr /* Make changes effective */
+1:
/* Setup cr0 */
movl $CR0_STATE, %eax
/* Make changes effective */
diff --git a/arch/x86/realmode/rm/trampoline_64.S b/arch/x86/realmode/rm/trampoline_64.S
index ae112a91592f..170f248d5769 100644
--- a/arch/x86/realmode/rm/trampoline_64.S
+++ b/arch/x86/realmode/rm/trampoline_64.S
@@ -143,13 +143,28 @@ SYM_CODE_START(startup_32)
movl %eax, %cr3

# Set up EFER
+ movl $MSR_EFER, %ecx
+ rdmsr
+ /*
+ * Skip writing to EFER if the register already has desired
+ * value (to avoid #VE for the TDX guest).
+ */
+ cmp pa_tr_efer, %eax
+ jne .Lwrite_efer
+ cmp pa_tr_efer + 4, %edx
+ je .Ldone_efer
+.Lwrite_efer:
movl pa_tr_efer, %eax
movl pa_tr_efer + 4, %edx
- movl $MSR_EFER, %ecx
wrmsr

- # Enable paging and in turn activate Long Mode
- movl $(X86_CR0_PG | X86_CR0_WP | X86_CR0_PE), %eax
+.Ldone_efer:
+ /*
+ * Enable paging and in turn activate Long Mode. Keep CR0.NE set, FERR#
+ * is no longer used to handle x87 FPU errors and clearing NE may fault
+ * in some environments.
+ */
+ movl $(X86_CR0_PG | X86_CR0_WP | X86_CR0_NE | X86_CR0_PE), %eax
movl %eax, %cr0

/*
@@ -169,7 +184,11 @@ SYM_CODE_START(pa_trampoline_compat)
movl $rm_stack_end, %esp
movw $__KERNEL_DS, %dx

- movl $X86_CR0_PE, %eax
+ /*
+ * Keep CR0.NE set, FERR# is no longer used to handle x87 FPU errors
+ * and clearing NE may fault in some environments.
+ */
+ movl $(X86_CR0_NE | X86_CR0_PE), %eax
movl %eax, %cr0
ljmpl $__KERNEL32_CS, $pa_startup_32
SYM_CODE_END(pa_trampoline_compat)
--
2.34.1

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