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@xxxxxxxxxxxx> > > --- > > 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 > >
