From 31c66dabaa8a48792469cd2d19a000a0b630aab4 Mon Sep 17 00:00:00 2001 From: Hou Wenlong Date: Tue, 8 Feb 2022 17:34:04 +0800 Subject: [PATCH] KVM: x86/emulator: Fix wrong privilege check for code segment in __load_segment_descriptor() 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 missing RPL < CPL check for far return. However, change code to follow the manual is better. Reviewed-by: Sean Christopherson Signed-off-by: Hou Wenlong Message-Id: Signed-off-by: Paolo Bonzini --- arch/x86/kvm/emulate.c | 31 +++++++++++++++++++++++-------- 1 file changed, 23 insertions(+), 8 deletions(-) diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index a6c54b8fdbc7..35e69c2adec2 100644 --- a/arch/x86/kvm/emulate.c +++ b/arch/x86/kvm/emulate.c @@ -1623,14 +1623,29 @@ 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; + /* RET can never return to an inner privilege level. */ + if (transfer == X86_TRANSFER_RET && rpl < cpl) + goto exception; + if (transfer == X86_TRANSFER_RET || transfer == 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) { -- 2.11.0