Subject: Re: [PATCH 3/3 V13] RO/NX protection for loadable kernel

From: Steven Rostedt <[email-masked]>
Date: Mon, 29 Nov 2010 13:15:42 -0500

This patch breaks function tracer:

------------[ cut here ]------------
WARNING: at kernel/trace/ftrace.c:1014 ftrace_bug+0x114/0x171()
Hardware name: Precision WorkStation 470    
Modules linked in: i2c_core(+)
Pid: 86, comm: modprobe Not tainted 2.6.37-rc2+ #68
Call Trace:
 [<ffffffff8104e957>] warn_slowpath_common+0x85/0x9d
 [<ffffffffa00026db>] ? __process_new_adapter+0x7/0x34 [i2c_core]
 [<ffffffffa00026db>] ? __process_new_adapter+0x7/0x34 [i2c_core]
 [<ffffffff8104e989>] warn_slowpath_null+0x1a/0x1c
 [<ffffffff810a9dfe>] ftrace_bug+0x114/0x171
 [<ffffffffa00026db>] ? __process_new_adapter+0x7/0x34 [i2c_core]
 [<ffffffff810aa0db>] ftrace_process_locs+0x1ae/0x274
 [<ffffffffa00026db>] ? __process_new_adapter+0x7/0x34 [i2c_core]
 [<ffffffff810aa29e>] ftrace_module_notify+0x39/0x44
 [<ffffffff814405cf>] notifier_call_chain+0x37/0x63
 [<ffffffff8106e054>] __blocking_notifier_call_chain+0x46/0x5b
 [<ffffffff8106e07d>] blocking_notifier_call_chain+0x14/0x16
 [<ffffffff8107ffde>] sys_init_module+0x73/0x1f3
 [<ffffffff8100acf2>] system_call_fastpath+0x16/0x1b
---[ end trace 2aff4f4ca53ec746 ]---
ftrace faulted on writing [<ffffffffa00026db>]
__process_new_adapter+0x7/0x34 [i2c_core]

On Tue, Nov 16, 2010 at 10:35:16PM +0100, matthieu castet wrote:
>  
> @@ -2650,6 +2810,18 @@ static struct module *load_module(void __user *umod,
>  	kfree(info.strmap);
>  	free_copy(&info);
>  
> +	/* Set RO and NX regions for core */
> +	set_section_ro_nx(mod->module_core,
> +				mod->core_text_size,
> +				mod->core_ro_size,
> +				mod->core_size);
> +
> +	/* Set RO and NX regions for init */
> +	set_section_ro_nx(mod->module_init,
> +				mod->init_text_size,
> +				mod->init_ro_size,
> +				mod->init_size);
> +

Here we set the text read only before we call the notifiers. The
function tracer changes the calls to mcount into nops via a notifier
call so this must be done after the module notifiers.

>  	/* Done! */
>  	trace_module_load(mod);
>  	return mod;
> @@ -2753,6 +2925,7 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
>  	mod->symtab = mod->core_symtab;
>  	mod->strtab = mod->core_strtab;
>  #endif
> +	unset_section_ro_nx(mod, mod->module_init);
>  	module_free(mod, mod->module_init);
>  	mod->module_init = NULL;
>  	mod->init_size = 0;

Below is the patch that fixes this bug.

-- Steve

Reported-by: Peter Zijlstra <[email-masked]>
Signed-off-by: Steven Rostedt <[email-masked]>

diff --git a/kernel/module.c b/kernel/module.c
index ba421e6..5ccf52c 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2804,18 +2804,6 @@ static struct module *load_module(void __user *umod,
 	kfree(info.strmap);
 	free_copy(&info);
 
-	/* Set RO and NX regions for core */
-	set_section_ro_nx(mod->module_core,
-				mod->core_text_size,
-				mod->core_ro_size,
-				mod->core_size);
-
-	/* Set RO and NX regions for init */
-	set_section_ro_nx(mod->module_init,
-				mod->init_text_size,
-				mod->init_ro_size,
-				mod->init_size);
-
 	/* Done! */
 	trace_module_load(mod);
 	return mod;
@@ -2876,6 +2864,18 @@ SYSCALL_DEFINE3(init_module, void __user *, umod,
 	blocking_notifier_call_chain(&module_notify_list,
 			MODULE_STATE_COMING, mod);
 
+	/* Set RO and NX regions for core */
+	set_section_ro_nx(mod->module_core,
+				mod->core_text_size,
+				mod->core_ro_size,
+				mod->core_size);
+
+	/* Set RO and NX regions for init */
+	set_section_ro_nx(mod->module_init,
+				mod->init_text_size,
+				mod->init_ro_size,
+				mod->init_size);
+
 	do_mod_ctors(mod);
 	/* Start the module */
 	if (mod->init != NULL)