[tip: sched/urgent] rseq: Set rseq::cpu_id_start to 0 on unregistration

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     2cb68e45120dfc66404c7547d95b8ac6ff0b25ce
Gitweb:        https://git.kernel.org/tip/2cb68e45120dfc66404c7547d95b8ac6ff0b25ce
Author:        Thomas Gleixner <tglx@xxxxxxxxxx>
AuthorDate:    Tue, 28 Apr 2026 10:10:19 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 01 May 2026 21:32:20 +02:00

rseq: Set rseq::cpu_id_start to 0 on unregistration

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

Fixes: 0f085b41880e ("rseq: Provide and use rseq_set_ids()")
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Tested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Link: https://patch.msgid.link/20260428224427.271566313%40kernel.org
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/rseq.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 38d3ef5..b9f1193 100644
--- a/kernel/rseq.c
+++ b/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;
 }