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

1 files changed, 11 insertions, 0 deletions

diff --git a/pkg/ring0/kernel_amd64.go b/pkg/ring0/kernel_amd64.go
index 92d2330cb..6e17fb796 100644
--- a/pkg/ring0/kernel_amd64.go
+++ b/pkg/ring0/kernel_amd64.go
@@ -250,6 +250,7 @@ func (c *CPU) SwitchToUser(switchOpts SwitchOpts) (vector Vector) {
 	}
 	SaveFloatingPoint(switchOpts.FloatingPointState.BytePointer()) // escapes: no. Copy out floating point.
 	WriteFS(uintptr(c.registers.Fs_base))                          // escapes: no. Restore kernel FS.
+	ldmxcsr(&kernelMXCSR)                                          // escapes: no. Restore kernel MXCSR.
 	return
 }
 
@@ -321,3 +322,13 @@ func SetCPUIDFaulting(on bool) bool {
 func ReadCR2() uintptr {
 	return readCR2()
 }
+
+// kernelMXCSR is the value of the mxcsr register in the Sentry.
+//
+// The MXCSR control configuration is initialized once and never changed. Look
+// at src/cmd/compile/abi-internal.md in the golang sources for more details.
+var kernelMXCSR uint32
+
+func init() {
+	stmxcsr(&kernelMXCSR)
+}