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

[Dmitry Vyukov]

On Wed, 29 Apr 2026 at 01:33, Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>
> 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>
> Cc: stable@xxxxxxxxxxxxxxx

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> ---
>  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;
>  }
>