Re: [PATCH 3/4] extable: Enable RCU if it is not watching in kernel_text_address()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Sep 22, 2017 at 06:15:46PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> If kernel_text_address() is called when RCU is not watching, it can cause an
> RCU bug because is_module_text_address() and the is_kprobe_*insn_slot()
> functions require the use of RCU.
> 
> Only enable RCU if it is not currently watching before it calls
> is_module_text_address(). The use of rcu_nmi_enter() is used to enable RCU
> because kernel_text_address() can happen pretty much anywhere (like an NMI),
> and even from within an NMI. It is called via save_stack_trace() that can be
> called by any WARN() or tracing function, which can happen while RCU is not
> watching (for example, going to or coming from idle, or during CPU take down
> or bring up).
> 
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: stable@vger.kernel.org

Do we need something calling out the dependency on the first two patches,
or is that implied these days?

> Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Give or take Josh's feedback on the commit log and comment:

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> ---
>  kernel/extable.c | 35 ++++++++++++++++++++++++++++++-----
>  1 file changed, 30 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/extable.c b/kernel/extable.c
> index a7024a494faf..89c4668376a6 100644
> --- a/kernel/extable.c
> +++ b/kernel/extable.c
> @@ -119,17 +119,42 @@ int __kernel_text_address(unsigned long addr)
> 
>  int kernel_text_address(unsigned long addr)
>  {
> +	bool no_rcu;
> +	int ret = 1;
> +
>  	if (core_kernel_text(addr))
>  		return 1;
> +
> +	/*
> +	 * If a stack dump happens while RCU is not watching, then
> +	 * RCU needs to be notified that it requires to start
> +	 * watching again. This can happen either by tracing that
> +	 * triggers a stack trace, or a WARN() that happens during
> +	 * coming back from idle, or cpu on or offlining.
> +	 *
> +	 * is_module_text_address() as well as the kprobe slots
> +	 * require RCU to be watching.
> +	 */
> +	no_rcu = !rcu_is_watching();
> +
> +	/* Treat this like an NMI as it can happen anywhere */
> +	if (no_rcu)
> +		rcu_nmi_enter();
> +
>  	if (is_module_text_address(addr))
> -		return 1;
> +		goto out;
>  	if (is_ftrace_trampoline(addr))
> -		return 1;
> +		goto out;
>  	if (is_kprobe_optinsn_slot(addr) || is_kprobe_insn_slot(addr))
> -		return 1;
> +		goto out;
>  	if (is_bpf_text_address(addr))
> -		return 1;
> -	return 0;
> +		goto out;
> +	ret = 0;
> +out:
> +	if (no_rcu)
> +		rcu_nmi_exit();
> +
> +	return ret;
>  }
> 
>  /*
> -- 
> 2.13.2
> 
>