arch/x86/kvm/emulate.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
When loading code segment descriptor in iret instruction emulation, the
checks are same as far return instruction emulation, so transfer type
should be X86_TRANSFER_RET in __load_segment_descriptor(). Although,
only iret in real mode is implemented now, and no checks are actually
needed for real mode, it would still be better to mark transfer type as
X86_TRANSFER_RET.
No functional change intended.
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
---
arch/x86/kvm/emulate.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 3b27622d4642..5052eb480068 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -2100,6 +2100,7 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt)
X86_EFLAGS_FIXED;
unsigned long vm86_mask = X86_EFLAGS_VM | X86_EFLAGS_VIF |
X86_EFLAGS_VIP;
+ u8 cpl = ctxt->ops->cpl(ctxt);
/* TODO: Add stack limit check */
@@ -2121,7 +2122,8 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt)
if (rc != X86EMUL_CONTINUE)
return rc;
- rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS);
+ rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl,
+ X86_TRANSFER_RET, NULL);
if (rc != X86EMUL_CONTINUE)
return rc;
--
2.31.1On Wed, Oct 12, 2022, Hou Wenlong wrote:
> When loading code segment descriptor in iret instruction emulation, the
> checks are same as far return instruction emulation, so transfer type
> should be X86_TRANSFER_RET in __load_segment_descriptor(). Although,
> only iret in real mode is implemented now, and no checks are actually
> needed for real mode, it would still be better to mark transfer type as
> X86_TRANSFER_RET.
It's not strictly a RET though. The RPL vs. DPL checks in __load_segment_descriptor()
might do the right thing, but there's a rather large pile of stuff IRET can do that
RET can't (ignoring the fact that KVM doesn't even emulate FAR RET to outer privilege
levels).
And __emulate_int_real() also loads CS with X86_TRANSFER_NONE, i.e. KVM still has
a weird path to worry about.
Rather than make the IRET case slightly less wrong, what about adding a sanity
check in __load_segment_descriptor() that KVM doesn't attempt to load CS in Protected
Mode with X86_TRANSFER_NONE?
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 3b27622d4642..fe735e18c419 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1641,6 +1641,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
goto exception;
break;
case VCPU_SREG_CS:
+ /*
+ * KVM uses "none" when loading CS as part of emulating Real
+ * Mode exceptions and IRET (handled above). In all other
+ * cases, loading CS without a control transfer is a KVM bug.
+ */
+ if (WARN_ON_ONCE(transfer == X86_TRANSFER_NONE))
+ goto exception;
+
if (!(seg_desc.type & 8))
goto exception;
>
> No functional change intended.
>
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---
> arch/x86/kvm/emulate.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 3b27622d4642..5052eb480068 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -2100,6 +2100,7 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt)
> X86_EFLAGS_FIXED;
> unsigned long vm86_mask = X86_EFLAGS_VM | X86_EFLAGS_VIF |
> X86_EFLAGS_VIP;
> + u8 cpl = ctxt->ops->cpl(ctxt);
>
> /* TODO: Add stack limit check */
>
> @@ -2121,7 +2122,8 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt)
> if (rc != X86EMUL_CONTINUE)
> return rc;
>
> - rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS);
> + rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl,
> + X86_TRANSFER_RET, NULL);
>
> if (rc != X86EMUL_CONTINUE)
> return rc;
> --
> 2.31.1
>
On Thu, Oct 13, 2022 at 12:34:57AM +0800, Sean Christopherson wrote: > On Wed, Oct 12, 2022, Hou Wenlong wrote: > > When loading code segment descriptor in iret instruction emulation, the > > checks are same as far return instruction emulation, so transfer type > > should be X86_TRANSFER_RET in __load_segment_descriptor(). Although, > > only iret in real mode is implemented now, and no checks are actually > > needed for real mode, it would still be better to mark transfer type as > > X86_TRANSFER_RET. > > It's not strictly a RET though. The RPL vs. DPL checks in __load_segment_descriptor() > might do the right thing, but there's a rather large pile of stuff IRET can do that > RET can't (ignoring the fact that KVM doesn't even emulate FAR RET to outer privilege > levels). > Yes, if EFLAGS.NT is set, IRET performs a task switch instead of far return in Protected Mode. > And __emulate_int_real() also loads CS with X86_TRANSFER_NONE, i.e. KVM still has > a weird path to worry about. > > Rather than make the IRET case slightly less wrong, what about adding a sanity > check in __load_segment_descriptor() that KVM doesn't attempt to load CS in Protected > Mode with X86_TRANSFER_NONE? > OK, sanity check is needed if someone wants to implement IRET in Protected Mode but use load_segment_descriptor(). > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > index 3b27622d4642..fe735e18c419 100644 > --- a/arch/x86/kvm/emulate.c > +++ b/arch/x86/kvm/emulate.c > @@ -1641,6 +1641,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt, > goto exception; > break; > case VCPU_SREG_CS: > + /* > + * KVM uses "none" when loading CS as part of emulating Real > + * Mode exceptions and IRET (handled above). In all other > + * cases, loading CS without a control transfer is a KVM bug. > + */ > + if (WARN_ON_ONCE(transfer == X86_TRANSFER_NONE)) > + goto exception; > + > if (!(seg_desc.type & 8)) > goto exception; > Do I need to prepare this patch or you will add this directly? > > > > No functional change intended. > > > > Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com> > > --- > > arch/x86/kvm/emulate.c | 4 +++- > > 1 file changed, 3 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > > index 3b27622d4642..5052eb480068 100644 > > --- a/arch/x86/kvm/emulate.c > > +++ b/arch/x86/kvm/emulate.c > > @@ -2100,6 +2100,7 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt) > > X86_EFLAGS_FIXED; > > unsigned long vm86_mask = X86_EFLAGS_VM | X86_EFLAGS_VIF | > > X86_EFLAGS_VIP; > > + u8 cpl = ctxt->ops->cpl(ctxt); > > > > /* TODO: Add stack limit check */ > > > > @@ -2121,7 +2122,8 @@ static int emulate_iret_real(struct x86_emulate_ctxt *ctxt) > > if (rc != X86EMUL_CONTINUE) > > return rc; > > > > - rc = load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS); > > + rc = __load_segment_descriptor(ctxt, (u16)cs, VCPU_SREG_CS, cpl, > > + X86_TRANSFER_RET, NULL); > > > > if (rc != X86EMUL_CONTINUE) > > return rc; > > -- > > 2.31.1 > >
On Thu, Oct 13, 2022, Hou Wenlong wrote: > On Thu, Oct 13, 2022 at 12:34:57AM +0800, Sean Christopherson wrote: > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c > > index 3b27622d4642..fe735e18c419 100644 > > --- a/arch/x86/kvm/emulate.c > > +++ b/arch/x86/kvm/emulate.c > > @@ -1641,6 +1641,14 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt, > > goto exception; > > break; > > case VCPU_SREG_CS: > > + /* > > + * KVM uses "none" when loading CS as part of emulating Real > > + * Mode exceptions and IRET (handled above). In all other > > + * cases, loading CS without a control transfer is a KVM bug. > > + */ > > + if (WARN_ON_ONCE(transfer == X86_TRANSFER_NONE)) > > + goto exception; > > + > > if (!(seg_desc.type & 8)) > > goto exception; > > > Do I need to prepare this patch or you will add this directly? No preference. Feel free to post a patch, if not I'll get to it soon-ish.
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