platform/kvm/x86: restore mxcsr when switching from guest to sentry

Goruntime sets mxcsr once and never changes it.

Reported-by: syzbot+ec55cea6e57ec083b7a6@syzkaller.appspotmail.com
Fixes: #5754

3 files changed, 52 insertions, 0 deletions

diff --git a/pkg/sentry/platform/kvm/BUILD b/pkg/sentry/platform/kvm/BUILD
index f04898dc1..47b294312 100644
--- a/pkg/sentry/platform/kvm/BUILD
+++ b/pkg/sentry/platform/kvm/BUILD
@@ -66,6 +66,7 @@ go_test(
     srcs = [
         "kvm_amd64_test.go",
         "kvm_arm64_test.go",
+        "kvm_amd64_test.s",
         "kvm_test.go",
         "virtual_map_test.go",
     ],
diff --git a/pkg/sentry/platform/kvm/kvm_amd64_test.go b/pkg/sentry/platform/kvm/kvm_amd64_test.go
index e44e995a0..382953cf7 100644
--- a/pkg/sentry/platform/kvm/kvm_amd64_test.go
+++ b/pkg/sentry/platform/kvm/kvm_amd64_test.go
@@ -49,3 +49,33 @@ func TestSegments(t *testing.T) {
 		return false
 	})
 }
+
+// stmxcsr reads the MXCSR control and status register.
+func stmxcsr(addr *uint32)
+
+func TestMXCSR(t *testing.T) {
+	applicationTest(t, true, testutil.SyscallLoop, func(c *vCPU, regs *arch.Registers, pt *pagetables.PageTables) bool {
+		var si arch.SignalInfo
+		switchOpts := ring0.SwitchOpts{
+			Registers:          regs,
+			FloatingPointState: &dummyFPState,
+			PageTables:         pt,
+			FullRestore:        true,
+		}
+		mxcsrBefore := uint32(0)
+		mxcsrAfter := uint32(0)
+		stmxcsr(&mxcsrBefore)
+		switchOpts.FloatingPointState.SetMXCSR(mxcsrBefore ^ 0x8)
+		if _, err := c.SwitchToUser(
+			switchOpts, &si); err == platform.ErrContextInterrupt {
+			return true // Retry.
+		} else if err != nil {
+			t.Errorf("application syscall failed: %v", err)
+		}
+		stmxcsr(&mxcsrAfter)
+		if mxcsrAfter != mxcsrBefore {
+			t.Errorf("mxcsr = %x (expected %x)", mxcsrBefore, mxcsrAfter)
+		}
+		return false
+	})
+}
diff --git a/pkg/sentry/platform/kvm/kvm_amd64_test.s b/pkg/sentry/platform/kvm/kvm_amd64_test.s
new file mode 100644
index 000000000..8e9079867
--- /dev/null
+++ b/pkg/sentry/platform/kvm/kvm_amd64_test.s
@@ -0,0 +1,21 @@
+// Copyright 2021 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "textflag.h"
+
+// stmxcsr reads the MXCSR control and status register.
+TEXT ·stmxcsr(SB),NOSPLIT,$0-8
+	MOVQ addr+0(FP), SI
+	STMXCSR (SI)
+	RET