signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed

As Andy pointed out that there are races between
force_sig_info_to_task and sigaction[1] when force_sig_info_task.  As
Kees discovered[2] ptrace is also able to change these signals.

In the case of seeccomp killing a process with a signal it is a
security violation to allow the signal to be caught or manipulated.

Solve this problem by introducing a new flag SA_IMMUTABLE that
prevents sigaction and ptrace from modifying these forced signals.
This flag is carefully made kernel internal so that no new ABI is
introduced.

Longer term I think this can be solved by guaranteeing short circuit
delivery of signals in this case.  Unfortunately reliable and
guaranteed short circuit delivery of these signals is still a ways off
from being implemented, tested, and merged.  So I have implemented a much
simpler alternative for now.

[1] https://lkml.kernel.org/r/b5d52d25-7bde-4030-a7b1-7c6f8ab90660@www.fastmail.com
[2] https://lkml.kernel.org/r/202110281136.5CE65399A7@keescook
Cc: stable@vger.kernel.org
Fixes: 307d522f ("signal/seccomp: Refactor seccomp signal and coredump generation")
Tested-by: Andrea Righi <andrea.righi@canonical.com>
Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

include/linux/signal_types.h

@@ -70,6 +70,9 @@ struct ksignal {
 	int sig;
 };
 
+/* Used to kill the race between sigaction and forced signals */
+#define SA_IMMUTABLE		0x00800000
+
 #ifndef __ARCH_UAPI_SA_FLAGS
 #ifdef SA_RESTORER
 #define __ARCH_UAPI_SA_FLAGS	SA_RESTORER

include/uapi/asm-generic/signal-defs.h

@@ -45,6 +45,7 @@
 #define SA_UNSUPPORTED	0x00000400
 #define SA_EXPOSE_TAGBITS	0x00000800
 /* 0x00010000 used on mips */
+/* 0x00800000 used for internal SA_IMMUTABLE */
 /* 0x01000000 used on x86 */
 /* 0x02000000 used on x86 */
 /*

kernel/signal.c

@@ -1336,6 +1336,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
 	blocked = sigismember(&t->blocked, sig);
 	if (blocked || ignored || sigdfl) {
 		action->sa.sa_handler = SIG_DFL;
+		action->sa.sa_flags |= SA_IMMUTABLE;
 		if (blocked) {
 			sigdelset(&t->blocked, sig);
 			recalc_sigpending_and_wake(t);
@@ -2760,7 +2761,8 @@ bool get_signal(struct ksignal *ksig)
 		if (!signr)
 			break; /* will return 0 */
 
-		if (unlikely(current->ptrace) && signr != SIGKILL) {
+		if (unlikely(current->ptrace) && (signr != SIGKILL) &&
+		    !(sighand->action[signr -1].sa.sa_flags & SA_IMMUTABLE)) {
 			signr = ptrace_signal(signr, &ksig->info);
 			if (!signr)
 				continue;
@@ -4110,6 +4112,10 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
 	k = &p->sighand->action[sig-1];
 
 	spin_lock_irq(&p->sighand->siglock);
+	if (k->sa.sa_flags & SA_IMMUTABLE) {
+		spin_unlock_irq(&p->sighand->siglock);
+		return -EINVAL;
+	}
 	if (oact)
 		*oact = *k;