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

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>

diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index 235c4af6b692a4618ad24e530eb3a19672f69a38..98bfa2e00d88ae9890687ac7d1488a0c3a8f22ae 100644 (file)
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -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. */