KVM: x86: Mark TSS busy during LTR emulation _after_ all fault checks

Wait to mark the TSS as busy during LTR emulation until after all fault
checks for the LTR have passed.  Specifically, don't mark the TSS busy if
the new TSS base is non-canonical.

Opportunistically drop the one-off !seg_desc.PRESENT check for TR as the
only reason for the early check was to avoid marking a !PRESENT TSS as
busy, i.e. the common !PRESENT is now done before setting the busy bit.

Fixes: 2268660cef03 ("KVM: x86: Emulator ignores LDTR/TR extended base on LLDT/LTR")
Reported-by: syzbot+760a73552f47a8cd0fd9@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20220711232750.1092012-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 39ea9138224c62b434b917b77ed4cd66662ecc55..09e4b67b881fca02d3a57afa77e69cc2ae405f0b 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -1699,16 +1699,6 @@ 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,
-                                                 sizeof(seg_desc), &ctxt->exception);
-               if (ret != X86EMUL_CONTINUE)
-                       return ret;
                break;
        case VCPU_SREG_LDTR:
                if (seg_desc.s || seg_desc.type != 2)
@@ -1749,6 +1739,15 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
                                ((u64)base3 << 32), ctxt))
                        return emulate_gp(ctxt, 0);
        }
+
+       if (seg == VCPU_SREG_TR) {
+               old_desc = seg_desc;
+               seg_desc.type |= 2; /* busy */
+               ret = ctxt->ops->cmpxchg_emulated(ctxt, desc_addr, &old_desc, &seg_desc,
+                                                 sizeof(seg_desc), &ctxt->exception);
+               if (ret != X86EMUL_CONTINUE)
+                       return ret;
+       }
 load:
        ctxt->ops->set_segment(ctxt, selector, &seg_desc, base3, seg);
        if (desc)