[patch 01/10] rseq: Set rseq::cpu_id_start to 0 on unregistration

Thomas Gleixner
2026-04-28T23:33:41Z

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@google.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
---
kernel/rseq.c | 20 +++++++++-----------
1 file changed, 9 insertions(+), 11 deletions(-)

--- 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;
}