Re: [PATCH] RO/NX protection for loadable kernel modules

[Siarhei Liakh]

On Tue, Jun 23, 2009 at 6:00 AM, Ingo Molnar<mingo@xxxxxxx> wrote:
>
> * James Morris <jmorris@xxxxxxxxx> wrote:
>
>> On Thu, 18 Jun 2009, Siarhei Liakh wrote:
>>
>> > This patch is a logical extension of the protection provided by
>> > CONFIG_DEBUG_RODATA to LKMs. The protection is provided by splitting
>> > module_core and module_init into three logical parts each and setting
>> > appropriate page access permissions for each individual section:
>> >
>> >    1. Code: RO+X
>> >    2. RO data: RO+NX
>> >    3. RW data: RW+NX
>> >
>> > In order to achieve proper protection, layout_sections() have been
>> > modified to align each of the three parts mentioned above onto page
>> > boundary. Next, the corresponding page access permissions are set
>> > right before successful exit from load_module(). Further,
>> > module_free() have been modified to set module_core or module_init as
>> > RW+NX right before calling vfree(). Functionality of this patch is
>> > enabled only when CONFIG_DEBUG_RODATA defined at compile time.
>> >
>>
>> This looks potentially useful to me, but I'm not an x86 expert
>> (several now added to Cc:).
>
> Pinged a few folks about this already. It looks useful, with the
> main worry being:
>
>  1) the increase in effective module size (probably worth the price)
>  2) some uglies in the patch (fixable)
>
> The main ugliness is the excessive use of #ifdefs - those should be
> eliminated. Also, most scripts/checkpatch.pl warnings about this
> patch should be taken seriously.
>
>        Ingo

1: You are correct. This patch effectively sets the lower limit of 3
pages for a module (well, there are some modules that do not have
.text and/or .data, but we are not talking about these extremes). So,
for small (sub-one-page) modules this will create 300% overhead.
However, for large modules the overhead is not that significant.

2: Agree. Will fix.

The main reason the #ifdefs are there is to make this patch dependent
on CONFIG_DEBUG_RODATA, since not everyone may be willing to pay 300%
overhead for the modules they use. I guess I could do some code
re-factoring, but #ifdefs will still be there. Or did you really mean
to eliminate all of them, making this patch a permanent feature of the
kernel? Can you please point me into right direction on how to
eliminate the #ifdefs while allowing to exclude the patch at compile
time if necessary?

Thanks.
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