[PATCH 1/2] KVM: x86/emulator: Defer not-present segment check in __load_segment_descriptor()

[PATCH 1/2] KVM: x86/emulator: Defer not-present segment check in __load_segment_descriptor()

[Hou Wenlong]

Per Intel's SDM on the "Instruction Set Reference", when
loading segment descriptor, not-present segment check should
be after all type and privilege checks. But the emulator checks
it first, then #NP is triggered instead of #GP if privilege fails
and segment is not present. Put not-present segment check after
type and privilege checks in __load_segment_descriptor().

Fixes: 38ba30ba51a00 (KVM: x86 emulator: Emulate task switch in emulator.c)
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
---
 arch/x86/kvm/emulate.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 166a145fc1e6..864db6fbe8db 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1616,11 +1616,6 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 		goto exception;
 	}

-	if (!seg_desc.p) {
-		err_vec = (seg == VCPU_SREG_SS) ? SS_VECTOR : NP_VECTOR;
-		goto exception;
-	}
-
 	dpl = seg_desc.dpl;

 	switch (seg) {
@@ -1660,6 +1655,10 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 	case VCPU_SREG_TR:
 		if (seg_desc.s || (seg_desc.type != 1 && seg_desc.type != 9))
 			goto exception;
+		if (!seg_desc.p) {
+			err_vec = NP_VECTOR;
+			goto exception;
+		}
 		old_desc = seg_desc;
 		seg_desc.type |= 2; /* busy */
 		ret = ctxt->ops->cmpxchg_emulated(ctxt, desc_addr, &old_desc, &seg_desc,
@@ -1684,6 +1683,11 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 		break;
 	}

+	if (!seg_desc.p) {
+		err_vec = (seg == VCPU_SREG_SS) ? SS_VECTOR : NP_VECTOR;
+		goto exception;
+	}
+
 	if (seg_desc.s) {
 		/* mark segment as accessed */
 		if (!(seg_desc.type & 1)) {
--
2.31.1

Re: [PATCH 1/2] KVM: x86/emulator: Defer not-present segment check in __load_segment_descriptor()

[Sean Christopherson]

On Thu, Jan 20, 2022, Hou Wenlong wrote:
> Per Intel's SDM on the "Instruction Set Reference", when
> loading segment descriptor, not-present segment check should
> be after all type and privilege checks. But the emulator checks
> it first, then #NP is triggered instead of #GP if privilege fails
> and segment is not present. Put not-present segment check after
> type and privilege checks in __load_segment_descriptor().

For posterity, KVM doesn't support CALL GATES or TASK GATES, so the "early" #NP
check for those is missing.

> Fixes: 38ba30ba51a00 (KVM: x86 emulator: Emulate task switch in emulator.c)
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

[PATCH 2/2] KVM: x86: Fix wrong privilege check for code segment in __load_segment_descriptor()

[Hou Wenlong]

Code segment descriptor can be loaded by jmp/call/ret, iret
and int. The privilege checks are different between those
instructions above realmode. Although, the emulator has
use x86_transfer_type enumerate to differentiate them, but
it is not really used in __load_segment_descriptor(). Note,
far jump/call to call gate, task gate or task state segment
are not implemented in emulator.

As for far jump/call to code segment, if DPL > CPL for conforming
code or (RPL > CPL or DPL != CPL) for non-conforming code, it
should trigger #GP. The current checks are ok.

As for far return, if RPL < CPL or DPL > RPL for conforming
code or DPL != RPL for non-conforming code, it should trigger #GP.
Outer level return is not implemented above virtual-8086 mode in
emulator. So it implies that RPL <= CPL, but the current checks
wouldn't trigger #GP if RPL < CPL.

As for code segment loading in task switch, if DPL > RPL for conforming
code or DPL != RPL for non-conforming code, it should trigger #TS. Since
segment selector is loaded before segment descriptor when load state from
tss, it implies that RPL = CPL, so the current checks are ok.

The only problem in current implementation is mssing RPL < CPL check for
far return. However, change code to follow the manual is better.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
---
 arch/x86/kvm/emulate.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 864db6fbe8db..b7ce2a85e58e 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1631,14 +1631,28 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 		if (!(seg_desc.type & 8))
 			goto exception;
 
-		if (seg_desc.type & 4) {
-			/* conforming */
-			if (dpl > cpl)
-				goto exception;
-		} else {
-			/* nonconforming */
-			if (rpl > cpl || dpl != cpl)
-				goto exception;
+		if (transfer == X86_TRANSFER_RET && rpl < cpl)
+			goto exception;
+		if (transfer == X86_TRANSFER_RET || X86_TRANSFER_TASK_SWITCH) {
+			if (seg_desc.type & 4) {
+				/* conforming */
+				if (dpl > rpl)
+					goto exception;
+			} else {
+				/* nonconforming */
+				if (dpl != rpl)
+					goto exception;
+			}
+		} else { /* X86_TRANSFER_CALL_JMP */
+			if (seg_desc.type & 4) {
+				/* conforming */
+				if (dpl > cpl)
+					goto exception;
+			} else {
+				/* nonconforming */
+				if (rpl > cpl || dpl != cpl)
+					goto exception;
+			}
 		}
 		/* in long-mode d/b must be clear if l is set */
 		if (seg_desc.d && seg_desc.l) {
-- 
2.31.1

Re: [PATCH 2/2] KVM: x86: Fix wrong privilege check for code segment in __load_segment_descriptor()

[Sean Christopherson]

On Thu, Jan 20, 2022, Hou Wenlong wrote:
> Code segment descriptor can be loaded by jmp/call/ret, iret
> and int. The privilege checks are different between those
> instructions above realmode. Although, the emulator has
> use x86_transfer_type enumerate to differentiate them, but
> it is not really used in __load_segment_descriptor(). Note,
> far jump/call to call gate, task gate or task state segment
> are not implemented in emulator.
> 
> As for far jump/call to code segment, if DPL > CPL for conforming
> code or (RPL > CPL or DPL != CPL) for non-conforming code, it
> should trigger #GP. The current checks are ok.
> 
> As for far return, if RPL < CPL or DPL > RPL for conforming
> code or DPL != RPL for non-conforming code, it should trigger #GP.
> Outer level return is not implemented above virtual-8086 mode in
> emulator. So it implies that RPL <= CPL, but the current checks
> wouldn't trigger #GP if RPL < CPL.
> 
> As for code segment loading in task switch, if DPL > RPL for conforming
> code or DPL != RPL for non-conforming code, it should trigger #TS. Since
> segment selector is loaded before segment descriptor when load state from
> tss, it implies that RPL = CPL, so the current checks are ok.
> 
> The only problem in current implementation is mssing RPL < CPL check for
> far return. However, change code to follow the manual is better.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

>  arch/x86/kvm/emulate.c | 30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 864db6fbe8db..b7ce2a85e58e 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1631,14 +1631,28 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
>  		if (!(seg_desc.type & 8))
>  			goto exception;
>  
> -		if (seg_desc.type & 4) {
> -			/* conforming */
> -			if (dpl > cpl)
> -				goto exception;
> -		} else {
> -			/* nonconforming */
> -			if (rpl > cpl || dpl != cpl)
> -				goto exception;

A comment here would be mildly helpful, e.g.

		/* RET can never return to an inner privilege level. */
> +		if (transfer == X86_TRANSFER_RET && rpl < cpl)
> +			goto exception;

And then as a follow-up patch, I think we can/should move the unhandled outer
privilege level logic here to make it easier to understand why the checks for RET
are incomplete, e.g.

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index a885b53dc7cc..a7cecd7beb91 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1631,8 +1631,15 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
                if (!(seg_desc.type & 8))
                        goto exception;

-               if (transfer == X86_TRANSFER_RET && rpl < cpl)
-                       goto exception;
+               if (transfer == X86_TRANSFER_RET) {
+                       /* RET can never return to an inner privilege level. */
+                       if (rpl < cpl)
+                               goto exception;
+                       /* Outer-privilege level return is not implemented */
+                       if (rpl > cpl)
+                               return X86EMUL_UNHANDLEABLE;
+               }
+
                if (transfer == X86_TRANSFER_RET || X86_TRANSFER_TASK_SWITCH) {
                        if (seg_desc.type & 4) {
                                /* conforming */
@@ -2227,9 +2234,6 @@ static int em_ret_far(struct x86_emulate_ctxt *ctxt)
        rc = emulate_pop(ctxt, &cs, ctxt->op_bytes);
        if (rc != X86EMUL_CONTINUE)
                return rc;
-       /* Outer-privilege level return is not implemented */
-       if (ctxt->mode >= X86EMUL_MODE_PROT16 && (cs & 3) > cpl)
-               return X86EMUL_UNHANDLEABLE;
        rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl,
                                       X86_TRANSFER_RET,
                                       &new_desc);