linux-kernel - Re: [PATCH] rseq: update kernel fields in lockstep with CONFIG_DEBUG

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Message-ID: <Z7nQzOQT_-9-Rbr5@gmail.com>
Date: Sat, 22 Feb 2025 14:27:40 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Michael Jeanson <mjeanson@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rseq: update kernel fields in lockstep with
 CONFIG_DEBUG_RSEQ


* Michael Jeanson <mjeanson@...icios.com> wrote:

> With CONFIG_DEBUG_RSEQ an in-kernel copy of the read-only fields is
> kept synchronized with the user-space fields. Ensure the updates
> are done in lockstep in case we error out on a write to user-space.
> 
> Fixes: 7d5265ffcd8b ("rseq: Validate read-only fields under DEBUG_RSEQ config")
> Signed-off-by: Michael Jeanson <mjeanson@...icios.com>
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> ---
>  kernel/rseq.c | 85 +++++++++++++++++++++++++++------------------------
>  1 file changed, 45 insertions(+), 40 deletions(-)
> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 2cb16091ec0a..5bdb96944e1f 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -26,6 +26,11 @@
>  				  RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL | \
>  				  RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE)
>  
> +static struct rseq __user *rseq_user_fields(struct task_struct *t)
> +{
> +	return (struct rseq __user *) t->rseq;
> +}
> +
>  #ifdef CONFIG_DEBUG_RSEQ
>  static struct rseq *rseq_kernel_fields(struct task_struct *t)
>  {
> @@ -78,24 +83,24 @@ static int rseq_validate_ro_fields(struct task_struct *t)
>  	return -EFAULT;
>  }
>  
> -static void rseq_set_ro_fields(struct task_struct *t, u32 cpu_id_start, u32 cpu_id,
> -			       u32 node_id, u32 mm_cid)
> -{
> -	rseq_kernel_fields(t)->cpu_id_start = cpu_id;
> -	rseq_kernel_fields(t)->cpu_id = cpu_id;
> -	rseq_kernel_fields(t)->node_id = node_id;
> -	rseq_kernel_fields(t)->mm_cid = mm_cid;
> -}
> +/*
> + * Update an rseq field and its in-kernel copy in lock-step to keep a coherent
> + * state.
> + */
> +#define unsafe_rseq_set_field(t, field, value, error_label)		\
> +	do {								\
> +		unsafe_put_user(value, &rseq_user_fields(t)->field, error_label);	\
> +		rseq_kernel_fields(t)->field = value;			\
> +	} while (0)
> +
>  #else
>  static int rseq_validate_ro_fields(struct task_struct *t)
>  {
>  	return 0;
>  }
>  
> -static void rseq_set_ro_fields(struct task_struct *t, u32 cpu_id_start, u32 cpu_id,
> -			       u32 node_id, u32 mm_cid)
> -{
> -}
> +#define unsafe_rseq_set_field(t, field, value, error_label)		\
> +	unsafe_put_user(value, &rseq_user_fields(t)->field, error_label)
>  #endif
>  
>  /*
> @@ -173,17 +178,18 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
>  	WARN_ON_ONCE((int) mm_cid < 0);
>  	if (!user_write_access_begin(rseq, t->rseq_len))
>  		goto efault;
> -	unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
> -	unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
> -	unsafe_put_user(node_id, &rseq->node_id, efault_end);
> -	unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
> +
> +	unsafe_rseq_set_field(t, cpu_id_start, cpu_id, efault_end);
> +	unsafe_rseq_set_field(t, cpu_id, cpu_id, efault_end);
> +	unsafe_rseq_set_field(t, node_id, node_id, efault_end);
> +	unsafe_rseq_set_field(t, mm_cid, mm_cid, efault_end);

Could we please name the new wrapper rseq_unsafe_put_user(), to make it 
clear it's a wrapper around unsafe_put_user()?

Thanks,

	Ingo