signal: don't silently convert SI_USER signals to non-current pidfd

The current sys_pidfd_send_signal() silently turns signals with explicit
SI_USER context that are sent to non-current tasks into signals with
kernel-generated siginfo.
This is unlike do_rt_sigqueueinfo(), which returns -EPERM in this case.
If a user actually wants to send a signal with kernel-provided siginfo,
they can do that with pidfd_send_signal(pidfd, sig, NULL, 0); so allowing
this case is unnecessary.

Instead of silently replacing the siginfo, just bail out with an error;
this is consistent with other interfaces and avoids special-casing behavior
based on security checks.

Fixes: 3eb39f47 ("signal: add pidfd_send_signal() syscall")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner <christian@brauner.io>

kernel/signal.c

@@ -3605,16 +3605,11 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
 		if (unlikely(sig != kinfo.si_signo))
 			goto err;
 
+		/* Only allow sending arbitrary signals to yourself. */
+		ret = -EPERM;
 		if ((task_pid(current) != pid) &&
-		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
-			/* Only allow sending arbitrary signals to yourself. */
-			ret = -EPERM;
-			if (kinfo.si_code != SI_USER)
-				goto err;
-
-			/* Turn this into a regular kill signal. */
-			prepare_kill_siginfo(sig, &kinfo);
-		}
+		    (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL))
+			goto err;
 	} else {
 		prepare_kill_siginfo(sig, &kinfo);
 	}