[PATCH v4 1/7] KVM: x86: Add an emulator flag to differentiate branch targets from fetches

[Sohil Mehta]

From: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

Add a new emulator flag, X86EMUL_F_BRANCH, and use it instead of
X86EMUL_F_FETCH in assign_eip() to distinguish between instruction fetch
and branch target computation for features that handle them differently.

For example, Linear Address Space Separation (LASS) applies to code
fetches but not branch target calculations. A LASS violation occurs only
when the branch target is used to fetch an instruction.

X86EMUL_F_BRANCH is still a code access, so add it to the code segment
readability check in __linearize() along with the Linear Address Masking
(LAM) untagging exemption in vmx_get_untagged_addr().

For now, X86EMUL_F_BRANCH and X86EMUL_F_FETCH are identical as far as
KVM is concerned. No functional change intended.

Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
---
v4:
 - Added LAM untagging exemption
 - Reworded the commit message
---
 arch/x86/kvm/emulate.c     | 5 +++--
 arch/x86/kvm/kvm_emulate.h | 1 +
 arch/x86/kvm/vmx/vmx.c     | 3 ++-
 3 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8071b372d233..8ff28643b2e3 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -672,7 +672,8 @@ static __always_inline int __linearize(struct x86_emulate_ctxt *ctxt,
 		    (flags & X86EMUL_F_WRITE))
 			goto bad;
 		/* unreadable code segment */
-		if (!(flags & X86EMUL_F_FETCH) && (desc.type & 8) && !(desc.type & 2))
+		if (!(flags & (X86EMUL_F_FETCH | X86EMUL_F_BRANCH)) &&
+		    (desc.type & 8) && !(desc.type & 2))
 			goto bad;
 		lim = desc_limit_scaled(&desc);
 		if (!(desc.type & 8) && (desc.type & 4)) {
@@ -723,7 +724,7 @@ static inline int assign_eip(struct x86_emulate_ctxt *ctxt, ulong dst)
 	if (ctxt->op_bytes != sizeof(unsigned long))
 		addr.ea = dst & ((1UL << (ctxt->op_bytes << 3)) - 1);
 	rc = __linearize(ctxt, addr, &max_size, 1, ctxt->mode, &linear,
-			 X86EMUL_F_FETCH);
+			 X86EMUL_F_BRANCH);
 	if (rc == X86EMUL_CONTINUE)
 		ctxt->_eip = addr.ea;
 	return rc;
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 3e375af15c03..97421b8dde13 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -105,6 +105,7 @@ struct x86_instruction_info {
 #define X86EMUL_F_INVLPG		BIT(3)
 #define X86EMUL_F_MSR			BIT(4)
 #define X86EMUL_F_DT_LOAD		BIT(5)
+#define X86EMUL_F_BRANCH		BIT(6)
 
 struct x86_emulate_ops {
 	void (*vm_bugged)(struct x86_emulate_ctxt *ctxt);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e3bfe6aca1a0..973f7e95be65 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8571,7 +8571,8 @@ gva_t vmx_get_untagged_addr(struct kvm_vcpu *vcpu, gva_t gva, unsigned int flags
 	int lam_bit;
 	unsigned long cr3_bits;
 
-	if (flags & (X86EMUL_F_FETCH | X86EMUL_F_IMPLICIT | X86EMUL_F_INVLPG))
+	if (flags & (X86EMUL_F_FETCH | X86EMUL_F_BRANCH | X86EMUL_F_IMPLICIT |
+		     X86EMUL_F_INVLPG))
 		return gva;
 
 	if (!is_64_bit_mode(vcpu))
-- 
2.43.0