Add a workaround to Linux 6.1's sev-guest lockout

The IV reuse fix 47894e0fa6a5 works against the host's throttling by
making the sev-guest device useless when throttled. The x86/urgent fix
72f7754dcf31 that merged to tip on March 16 still hasn't made it to
some distros, so this workaround is still needed temporarily.

Issue #40 is to revert this change at the appropriate time. The same
fix for #40 includes the fix for #5.

Signed-off-by: Dionna Glaze <[email protected]>

client/client_linux.go

@@ -19,17 +19,24 @@ package client
 
 import (
 	"fmt"
+	"time"
 
 	labi "github.com/google/go-sev-guest/client/linuxabi"
 	"golang.org/x/sys/unix"
 )
 
-// defaultSevGuestDevicePath is the platform's usual device path to the SEV guest.
-const defaultSevGuestDevicePath = "/dev/sev-guest"
+const (
+	// defaultSevGuestDevicePath is the platform's usual device path to the SEV guest.
+	defaultSevGuestDevicePath = "/dev/sev-guest"
+	throttleDuration          = 2 * time.Second
+	burstMax                  = 2
+)
 
 // LinuxDevice implements the Device interface with Linux ioctls.
 type LinuxDevice struct {
-	fd int
+	fd      int
+	lastCmd time.Time
+	burst   int
 }
 
 // Open opens the SEV-SNP guest device from a given path
@@ -71,11 +78,24 @@ func (d *LinuxDevice) Close() error {
 
 // Ioctl sends a command with its wrapped request and response values to the Linux device.
 func (d *LinuxDevice) Ioctl(command uintptr, req any) (uintptr, error) {
+	// TODO(Issue #40): Remove the workaround to the ENOTTY lockout when throttled
+	// in Linux 6.1 by throttling ourselves first.
+	if d.burst == 0 {
+		sinceLast := time.Since(d.lastCmd)
+		// Self-throttle for tests without guest OS throttle detection
+		if sinceLast < throttleDuration {
+			time.Sleep(throttleDuration - sinceLast)
+		}
+	}
 	switch sreq := req.(type) {
 	case *labi.SnpUserGuestRequest:
 		abi := sreq.ABI()
 		result, _, errno := unix.Syscall(unix.SYS_IOCTL, uintptr(d.fd), command, uintptr(abi.Pointer()))
 		abi.Finish(sreq)
+		d.burst = (d.burst + 1) % burstMax
+		if d.burst == 0 {
+			d.lastCmd = time.Now()
+		}
 
 		// TODO(Issue #5): remove the work around for the kernel bug that writes
 		// uninitialized memory back on non-EIO.