rseq: Set rseq::cpu_id_start to 0 on unregistration

commit 2cb68e4 upstream.

The RSEQ rework changed that to RSEQ_CPU_UNINITILIZED, which is obviously
incompatible. Revert back to the original behavior.

Fixes: 0f085b4 ("rseq: Provide and use rseq_set_ids()")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://patch.msgid.link/20260428224427.271566313%40kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Thomas Gleixner

kernel/rseq.c

@@ -236,11 +236,6 @@ static int __init rseq_debugfs_init(void)
 }
 __initcall(rseq_debugfs_init);
 
-static bool rseq_set_ids(struct task_struct *t, struct rseq_ids *ids, u32 node_id)
-{
-	return rseq_set_ids_get_csaddr(t, ids, node_id, NULL);
-}
-
 static bool rseq_handle_cs(struct task_struct *t, struct pt_regs *regs)
 {
 	struct rseq __user *urseq = t->rseq.usrptr;
@@ -384,19 +379,22 @@ void rseq_syscall(struct pt_regs *regs)
 
 static bool rseq_reset_ids(void)
 {
-	struct rseq_ids ids = {
-		.cpu_id		= RSEQ_CPU_ID_UNINITIALIZED,
-		.mm_cid		= 0,
-	};
+	struct rseq __user *rseq = current->rseq.usrptr;
 
 	/*
 	 * If this fails, terminate it because this leaves the kernel in
 	 * stupid state as exit to user space will try to fixup the ids
 	 * again.
 	 */
-	if (rseq_set_ids(current, &ids, 0))
-		return true;
+	scoped_user_rw_access(rseq, efault) {
+		unsafe_put_user(0, &rseq->cpu_id_start, efault);
+		unsafe_put_user(RSEQ_CPU_ID_UNINITIALIZED, &rseq->cpu_id, efault);
+		unsafe_put_user(0, &rseq->node_id, efault);
+		unsafe_put_user(0, &rseq->mm_cid, efault);
+	}
+	return true;
 
+efault:
 	force_sig(SIGSEGV);
 	return false;
 }