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Date: Fri, 26 May 2017 15:50:38 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
cc: Kees Cook <keescook@chromium.org>, LKML <linux-kernel@vger.kernel.org>,
        x86@kernel.org, Masami Hiramatsu <mhiramat@kernel.org>,
        "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH V2] x86/ftrace: Make sure that ftrace trampolines are
 not RWX
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On Fri, 26 May 2017, Steven Rostedt wrote:

> On Thu, 25 May 2017 10:57:51 +0200 (CEST)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > ftrace use module_alloc() to allocate trampoline pages. The mapping of
> > module_alloc() is RWX, which makes sense as the memory is written to right
> > after allocation. But nothing makes these pages RO after writing to them.
> > 
> > Add proper set_memory_rw/ro() calls to protect the trampolines after
> > modification.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > ---
> 
> Unfortunately, this was triggered in my tests:
> 
> ftrace: allocating 54840 entries in 215 pages
> Starting tracer 'function'
> ------------[ cut here ]------------
> kernel BUG at /work/autotest/nobackup/linux-test.git/arch/x86/mm/pageattr.c:189!
>
> It appears that if you enable function tracing at boot up, calling
> set_memory_ro() with interrupts disabled can cause this. As pageattr.c
> at line 189 has:
> 
> 	BUG_ON(irqs_disabled());

That's very early boot, right? So interrupts have to be disabled.

So this wants to be:

   BUG_ON(irqs_disabled() && !early_boot_irqs_disabled);

Thanks,

	tglx