linux-kernel - Re: [PATCH] ARM: ftrace: Only set kernel memory back to read-only after boot

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Message-ID: <c15a7eaba14073149a04d9aa649b10f9@agner.ch>
Date:   Tue, 10 Jul 2018 09:24:32 +0200
From:   Stefan Agner <stefan@...er.ch>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        abelvesa@...ux.com, Abhishek Sagar <sagar.abhishek@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] ARM: ftrace: Only set kernel memory back to read-only
 after boot

On 04.07.2018 13:30, Stefan Agner wrote:
> On 21.06.2018 18:47, Steven Rostedt wrote:
>> From: Steven Rostedt (VMware) <rostedt@...dmis.org>
>>
>> Dynamic ftrace requires modifying the code segments that are usually
>> set to read-only. To do this, a per arch function is called both before
>> and after the ftrace modifications are performed. The "before" function
>> will set kernel code text to read-write to allow for ftrace to make the
>> modifications, and the "after" function will set the kernel code text
>> back to "read-only" to keep the kernel code text protected.
>>
>> The issue happens when dynamic ftrace is tested at boot up. The test is
>> done before the kernel code text has been set to read-only. But the
>> "before" and "after" calls are still performed. The "after" call will
>> change the kernel code text to read-only prematurely, and other boot
>> code that expects this code to be read-write will fail.
>>
>> The solution is to add a variable that is set when the kernel code text
>> is expected to be converted to read-only, and make the ftrace "before"
>> and "after" calls do nothing if that variable is not yet set. This is
>> similar to the x86 solution from commit 162396309745 ("ftrace, x86:
>> make kernel text writable only for conversions").
>>
>> Reported-by: Stefan Agner <stefan@...er.ch>
>> Tested-by: Stefan Agner <stefan@...er.ch>
>> Link: http://lkml.kernel.org/r/20180620212906.24b7b66e@vmware.local.home
>> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
>> ---
>> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
>> index c186474422f3..0cc8e04295a4 100644
>> --- a/arch/arm/mm/init.c
>> +++ b/arch/arm/mm/init.c
> 
> I guess this can be seen as reviewed?
> 
> Patches touching core ARM port usually go through Russels patch tracker.
> 
> You have to submit the patch here:
> http://www.armlinux.org.uk/developer/patches/
> 

FWIW, I added this patch to the patch tracker, its patch ID is 8780/1.

--
Stefan

> 
> 
>> @@ -736,20 +736,29 @@ static int __mark_rodata_ro(void *unused)
>>  	return 0;
>>  }
>>
>> +static int kernel_set_to_readonly __read_mostly;
>> +
>>  void mark_rodata_ro(void)
>>  {
>> +	kernel_set_to_readonly = 1;
>>  	stop_machine(__mark_rodata_ro, NULL, NULL);
>>  	debug_checkwx();
>>  }
>>
>>  void set_kernel_text_rw(void)
>>  {
>> +	if (!kernel_set_to_readonly)
>> +		return;
>> +
>>  	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), false,
>>  				current->active_mm);
>>  }
>>
>>  void set_kernel_text_ro(void)
>>  {
>> +	if (!kernel_set_to_readonly)
>> +		return;
>> +
>>  	set_section_perms(ro_perms, ARRAY_SIZE(ro_perms), true,
>>  				current->active_mm);
>>  }