linux-kernel - Re: [PATCH V2] x86/ftrace: Make sure that ftrace trampolines are not RWX

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Date:   Thu, 25 May 2017 12:51:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V2] x86/ftrace: Make sure that ftrace trampolines are not RWX

On Thu, May 25, 2017 at 10:46 AM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
> On Thu, May 25, 2017 at 10:57:51AM +0200, Thomas Gleixner 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@...utronix.de>
>> ---
>>  arch/x86/kernel/ftrace.c |   20 ++++++++++++++------
>>  1 file changed, 14 insertions(+), 6 deletions(-)
>>
>> --- a/arch/x86/kernel/ftrace.c
>> +++ b/arch/x86/kernel/ftrace.c
>> @@ -689,8 +689,12 @@ static inline void *alloc_tramp(unsigned
>>  {
>>       return module_alloc(size);
>>  }
>> -static inline void tramp_free(void *tramp)
>> +static inline void tramp_free(void *tramp, int size)
>>  {
>> +     int npages = PAGE_ALIGN(size) >> PAGE_SHIFT;
>> +
>> +     set_memory_nx((unsigned long)tramp, npages);
>> +     set_memory_rw((unsigned long)tramp, npages);
>>       module_memfree(tramp);
>>  }
>
> Can/should module_memfree() just do this for users? With Masami's fix that'd
> be 2 users already.

It seems like it really should. That would put it in a single place
and avoid this mistake again in the future. Does module_memfree() have
access to the allocation size, or does that need to get plumbed?

-Kees

-- 
Kees Cook
Pixel Security