| Message ID | ed8917d7bab80a1c1a130beae45c7d6ecdef47fc.1642669684.git.houwenlong.hwl@antgroup.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86/emulator: Fix wrong checks when loading code segment in emulator | expand |
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);
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) {
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(-)