get_signal: hide_si_addr_tag_bits: fix the usage of uninitialized ksig

ksig->ka and ksig->info are not initialized if get_signal() returns 0 or
if the caller is PF_USER_WORKER.

Check signr != 0 before SA_EXPOSE_TAGBITS and move the "out" label down.

The latter means that ksig->sig won't be initialized if a PF_USER_WORKER
thread gets a fatal signal but this is fine, PF_USER_WORKER's don't use
ksig. And there is nothing new, in this case ksig->ka and ksig-info are
not initialized anyway. Add a comment.

Link: https://lkml.kernel.org/r/20240226165650.GA20829@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Wen Yang <wenyang.linux@foxmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: oleg-nesterov

kernel/signal.c

@@ -2881,8 +2881,9 @@ bool get_signal(struct ksignal *ksig)
 
 		/*
 		 * PF_USER_WORKER threads will catch and exit on fatal signals
-		 * themselves. They have cleanup that must be performed, so
-		 * we cannot call do_exit() on their behalf.
+		 * themselves. They have cleanup that must be performed, so we
+		 * cannot call do_exit() on their behalf. Note that ksig won't
+		 * be properly initialized, PF_USER_WORKER's shouldn't use it.
 		 */
 		if (current->flags & PF_USER_WORKER)
 			goto out;
@@ -2894,12 +2895,12 @@ bool get_signal(struct ksignal *ksig)
 		/* NOTREACHED */
 	}
 	spin_unlock_irq(&sighand->siglock);
-out:
+
 	ksig->sig = signr;
 
-	if (!(ksig->ka.sa.sa_flags & SA_EXPOSE_TAGBITS))
+	if (signr && !(ksig->ka.sa.sa_flags & SA_EXPOSE_TAGBITS))
 		hide_si_addr_tag_bits(ksig);
-
+out:
 	return signr > 0;
 }