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

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Date:   Thu, 21 Jun 2018 19:54:10 +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 21.06.2018 18:50, Steven Rostedt wrote:
> On Thu, 21 Jun 2018 12:47:10 -0400
> Steven Rostedt <rostedt@...dmis.org> 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>
>> ---
> 
> Perhaps I should have Cc'd stable too?
> 

As it is self tests only which are broken probably not super important.
But then, the fix is also rather clean and safe IMHO, so why not.

--
Stefan