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

Paul E. McKenney
2017-09-22T22:44:11Z

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
>
>