Re: [PATCH] stacktrace: Force USER_DS for stack_trace_save_user()

[Joel Fernandes]

On Thu, Jul 18, 2019 at 10:57:54AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 17, 2019 at 10:09:45AM +0200, Vegard Nossum wrote:
> > On 7/17/19 10:07 AM, Peter Zijlstra wrote:
> 
> > > Does something like the below help?
> 
> > Yes.
> 
> Thanks!
> 
> ---
> Subject: stacktrace: Force USER_DS for stack_trace_save_user()
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Thu Jul 18 10:47:47 CEST 2019
> 
> When walking userspace stacks, we should set USER_DS, otherwise
> access_ok() will not function as expected.
> 
> Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> Tested-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> Reported-by: Eiichi Tsukata <devel@xxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel


> ---
> --- a/kernel/stacktrace.c
> +++ b/kernel/stacktrace.c
> @@ -226,12 +226,17 @@ unsigned int stack_trace_save_user(unsig
>  		.store	= store,
>  		.size	= size,
>  	};
> +	mm_segment_t fs;
>  
>  	/* Trace user stack if not a kernel thread */
>  	if (current->flags & PF_KTHREAD)
>  		return 0;
>  
> +	fs = get_fs();
> +	set_fs(USER_DS);
>  	arch_stack_walk_user(consume_entry, &c, task_pt_regs(current));
> +	set_fs(fs);
> +
>  	return c.len;
>  }
>  #endif