linux-kernel - Re: [PATCH] ftrace: Remove possible deadlock between register_kprobe() and ftrace_run_update

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Date:   Thu, 27 Jun 2019 17:47:29 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Miroslav Benes <mbenes@...e.cz>, Jessica Yu <jeyu@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        Johannes Erdfelt <johannes@...felt.com>,
        Ingo Molnar <mingo@...nel.org>, mhiramat@...nel.org,
        torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [PATCH] ftrace: Remove possible deadlock between
 register_kprobe() and ftrace_run_update_code()

Thanks a lot for fixing this Petr.

On Thu, Jun 27, 2019 at 10:13:34AM +0200, Petr Mladek wrote:
> @@ -35,6 +36,7 @@
>  
>  int ftrace_arch_code_modify_prepare(void)
>  {
> +	mutex_lock(&text_mutex);
>  	set_kernel_text_rw();
>  	set_all_modules_text_rw();
>  	return 0;
> @@ -44,6 +46,7 @@ int ftrace_arch_code_modify_post_process(void)
>  {
>  	set_all_modules_text_ro();
>  	set_kernel_text_ro();
> +	mutex_unlock(&text_mutex);
>  	return 0;
>  }

Releasing the lock in a separate function seems a bit surprising and
fragile, would it be possible to do something like this instead?

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index b38c388d1087..89ea1af6fd13 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -37,15 +37,21 @@
 int ftrace_arch_code_modify_prepare(void)
 {
 	mutex_lock(&text_mutex);
+
 	set_kernel_text_rw();
 	set_all_modules_text_rw();
+
+	mutex_unlock(&text_mutex);
 	return 0;
 }
 
 int ftrace_arch_code_modify_post_process(void)
 {
+	mutex_lock(&text_mutex);
+
 	set_all_modules_text_ro();
 	set_kernel_text_ro();
+
 	mutex_unlock(&text_mutex);
 	return 0;
 }