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KVM: SVM: Ensure SPEC_CTRL[63:32] is context switched between guest and host

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SPEC_CTRL is an MSR, i.e. a 64-bit value, but the VMRUN assembly code
assumes bits 63:32 are always zero.  The bug is _currently_ benign because
neither KVM nor the kernel support setting any of bits 63:32, but it's
still a bug that needs to be fixed.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20251106191230.182393-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Uros Bizjak
2025-11-06 11:12:30 -08:00
committed by Sean Christopherson
parent 3d80f4c93d
commit c331b400e2
1 changed files with 37 additions and 10 deletions
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47
arch/x86/kvm/svm/vmenter.S
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@@ -52,11 +52,23 @@
* there must not be any returns or indirect branches between this code
* and vmentry.
*/
movl SVM_spec_ctrl(%_ASM_DI), %eax
cmp PER_CPU_VAR(x86_spec_ctrl_current), %eax
#ifdef CONFIG_X86_64
mov SVM_spec_ctrl(%rdi), %rdx
cmp PER_CPU_VAR(x86_spec_ctrl_current), %rdx
je 801b
movl %edx, %eax
shr $32, %rdx
#else
mov SVM_spec_ctrl(%edi), %eax
mov PER_CPU_VAR(x86_spec_ctrl_current), %ecx
xor %eax, %ecx
mov SVM_spec_ctrl + 4(%edi), %edx
mov PER_CPU_VAR(x86_spec_ctrl_current + 4), %esi
xor %edx, %esi
or %esi, %ecx
je 801b
#endif
mov $MSR_IA32_SPEC_CTRL, %ecx
xor %edx, %edx
wrmsr
jmp 801b
.endm
@@ -81,13 +93,25 @@
jnz 998f
rdmsr
movl %eax, SVM_spec_ctrl(%_ASM_DI)
movl %edx, SVM_spec_ctrl + 4(%_ASM_DI)
998:
/* Now restore the host value of the MSR if different from the guest's. */
movl PER_CPU_VAR(x86_spec_ctrl_current), %eax
cmp SVM_spec_ctrl(%_ASM_DI), %eax
#ifdef CONFIG_X86_64
mov PER_CPU_VAR(x86_spec_ctrl_current), %rdx
cmp SVM_spec_ctrl(%rdi), %rdx
je 901b
xor %edx, %edx
movl %edx, %eax
shr $32, %rdx
#else
mov PER_CPU_VAR(x86_spec_ctrl_current), %eax
mov SVM_spec_ctrl(%edi), %esi
xor %eax, %esi
mov PER_CPU_VAR(x86_spec_ctrl_current + 4), %edx
mov SVM_spec_ctrl + 4(%edi), %edi
xor %edx, %edi
or %edi, %esi
je 901b
#endif
wrmsr
jmp 901b
.endm
@@ -134,7 +158,7 @@ SYM_FUNC_START(__svm_vcpu_run)
mov %_ASM_ARG1, %_ASM_DI
.endif
/* Clobbers RAX, RCX, RDX. */
/* Clobbers RAX, RCX, RDX (and ESI on 32-bit), consumes RDI (@svm). */
RESTORE_GUEST_SPEC_CTRL
/*
@@ -211,7 +235,10 @@ SYM_FUNC_START(__svm_vcpu_run)
/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RSB_VMEXIT
/* Clobbers RAX, RCX, RDX. */
/*
* Clobbers RAX, RCX, RDX (and ESI, EDI on 32-bit), consumes RDI (@svm)
* and RSP (pointer to @spec_ctrl_intercepted).
*/
RESTORE_HOST_SPEC_CTRL
/*
@@ -331,7 +358,7 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
mov %rdi, SEV_ES_RDI (%rdx)
mov %rsi, SEV_ES_RSI (%rdx)
/* Clobbers RAX, RCX, RDX (@hostsa). */
/* Clobbers RAX, RCX, and RDX (@hostsa), consumes RDI (@svm). */
RESTORE_GUEST_SPEC_CTRL
/* Get svm->current_vmcb->pa into RAX. */