x86emul: avoid triggering event related assertions

The assertion at the end of x86_emulate_wrapper() as well as the ones
in x86_emul_{hw_exception,pagefault}() can trigger if we ignore
X86EMUL_EXCEPTION coming back from certain hook functions. Squash
exceptions when merely probing MSRs, plus on SWAPGS'es "best effort"
error handling path.

In adjust_bnd() add another assertion after the read_xcr(0, ...)
invocation, paralleling the one in x86emul_get_fpu() - XCR0 reads should
never fault when XSAVE is (implicitly) known to be available.

Also update the respective comment in x86_emulate_wrapper().

Fixes: 14a6be89ec ("x86emul: correct EFLAGS.TF handling")
Fixes: cb2626c758 ("x86emul: conditionally clear BNDn for branches")
Fixes: 6eb43fcf8a ("x86emul: support SWAPGS")
Reported-by: AFL
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/x86_emulate/0f01.c

@@ -200,8 +200,10 @@ int x86emul_0f01(struct x86_emulate_state *s,
         if ( (rc = ops->write_segment(x86_seg_gs, &sreg,
                                       ctxt)) != X86EMUL_OKAY )
         {
-            /* Best effort unwind (i.e. no error checking). */
-            ops->write_msr(MSR_SHADOW_GS_BASE, msr_val, ctxt);
+            /* Best effort unwind (i.e. no real error checking). */
+            if ( ops->write_msr(MSR_SHADOW_GS_BASE, msr_val,
+                                ctxt) == X86EMUL_EXCEPTION )
+                x86_emul_reset_event(ctxt);
             goto done;
         }
         break;

xen/arch/x86/x86_emulate/0fae.c

@@ -55,7 +55,10 @@ int x86emul_0fae(struct x86_emulate_state *s,
                     cr4 = X86_CR4_OSFXSR;
                 if ( !ops->read_msr ||
                      ops->read_msr(MSR_EFER, &msr_val, ctxt) != X86EMUL_OKAY )
+                {
+                    x86_emul_reset_event(ctxt);
                     msr_val = 0;
+                }
                 if ( !(cr4 & X86_CR4_OSFXSR) ||
                      (mode_64bit() && mode_ring0() && (msr_val & EFER_FFXSE)) )
                     s->op_bytes = offsetof(struct x86_fxsr, xmm[0]);

xen/arch/x86/x86_emulate/x86_emulate.c

@@ -1143,10 +1143,18 @@ static bool is_branch_step(struct x86_emulate_ctxt *ctxt,
                            const struct x86_emulate_ops *ops)
 {
     uint64_t debugctl;
+    int rc = X86EMUL_UNHANDLEABLE;
 
-    return ops->read_msr &&
-           ops->read_msr(MSR_IA32_DEBUGCTLMSR, &debugctl, ctxt) == X86EMUL_OKAY &&
-           (debugctl & IA32_DEBUGCTLMSR_BTF);
+    if ( !ops->read_msr ||
+         (rc = ops->read_msr(MSR_IA32_DEBUGCTLMSR, &debugctl,
+                             ctxt)) != X86EMUL_OKAY )
+    {
+        if ( rc == X86EMUL_EXCEPTION )
+            x86_emul_reset_event(ctxt);
+        debugctl = 0;
+    }
+
+    return debugctl & IA32_DEBUGCTLMSR_BTF;
 }
 
 static void adjust_bnd(struct x86_emulate_ctxt *ctxt,
@@ -1160,13 +1168,21 @@ static void adjust_bnd(struct x86_emulate_ctxt *ctxt,
 
     if ( !ops->read_xcr || ops->read_xcr(0, &xcr0, ctxt) != X86EMUL_OKAY ||
          !(xcr0 & X86_XCR0_BNDREGS) || !(xcr0 & X86_XCR0_BNDCSR) )
+    {
+        ASSERT(!ctxt->event_pending);
         return;
+    }
 
     if ( !mode_ring0() )
         bndcfg = read_bndcfgu();
     else if ( !ops->read_msr ||
-              ops->read_msr(MSR_IA32_BNDCFGS, &bndcfg, ctxt) != X86EMUL_OKAY )
+              (rc = ops->read_msr(MSR_IA32_BNDCFGS, &bndcfg,
+                                  ctxt)) != X86EMUL_OKAY )
+    {
+        if ( rc == X86EMUL_EXCEPTION )
+            x86_emul_reset_event(ctxt);
         return;
+    }
     if ( (bndcfg & IA32_BNDCFGS_ENABLE) && !(bndcfg & IA32_BNDCFGS_PRESERVE) )
     {
         /*
@@ -8677,7 +8693,9 @@ int x86_emulate_wrapper(
      * An event being pending should exactly match returning
      * X86EMUL_EXCEPTION.  (If this trips, the chances are a codepath has
      * called hvm_inject_hw_exception() rather than using
-     * x86_emul_hw_exception().)
+     * x86_emul_hw_exception(), or the invocation of a hook has caused an
+     * exception to be raised, while the caller was only checking for
+     * success/failure.)
      */
     ASSERT(ctxt->event_pending == (rc == X86EMUL_EXCEPTION));