Don't allow writing or reading to TTY unless process group is in foreground.

If a background process tries to read from a TTY, linux sends it a SIGTTIN
unless the signal is blocked or ignored, or the process group is an orphan, in
which case the syscall returns EIO.

See drivers/tty/n_tty.c:n_tty_read()=>job_control().

If a background process tries to write a TTY, set the termios, or set the
foreground process group, linux then sends a SIGTTOU. If the signal is ignored
or blocked, linux allows the write. If the process group is an orphan, the
syscall returns EIO.

See drivers/tty/tty_io.c:tty_check_change().

PiperOrigin-RevId: 234044367
Change-Id: I009461352ac4f3f11c5d42c43ac36bb0caa580f9

1 files changed, 8 insertions, 0 deletions

diff --git a/pkg/sentry/kernel/signal_handlers.go b/pkg/sentry/kernel/signal_handlers.go
index 3f1ac9898..60cbe85b8 100644
--- a/pkg/sentry/kernel/signal_handlers.go
+++ b/pkg/sentry/kernel/signal_handlers.go
@@ -69,6 +69,14 @@ func (sh *SignalHandlers) CopyForExec() *SignalHandlers {
 	return sh2
 }
 
+// IsIgnored returns true if the signal is ignored.
+func (sh *SignalHandlers) IsIgnored(sig linux.Signal) bool {
+	sh.mu.Lock()
+	defer sh.mu.Unlock()
+	sa, ok := sh.actions[sig]
+	return ok && sa.Handler == arch.SignalActIgnore
+}
+
 // dequeueActionLocked returns the SignalAct that should be used to handle sig.
 //
 // Preconditions: sh.mu must be locked.