[kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

[Thomas Garnier]

On Tue, May 9, 2017 at 4:10 AM, Greg KH <greg at kroah.com> wrote:
> On Tue, May 09, 2017 at 08:56:19AM +0200, Ingo Molnar wrote:
>>
>> * Kees Cook <keescook at chromium.org> wrote:
>>
>> > > There's the option of using GCC plugins now that the infrastructure was
>> > > upstreamed from grsecurity. It can be used as part of the regular build
>> > > process and as long as the analysis is pretty simple it shouldn't hurt compile
>> > > time much.
>> >
>> > Well, and that the situation may arise due to memory corruption, not from
>> > poorly-matched set_fs() calls, which static analysis won't help solve. We need
>> > to catch this bad kernel state because it is a very bad state to run in.
>>
>> If memory corruption corrupted the task state into having addr_limit set to
>> KERNEL_DS then there's already a fair chance that it's game over: it could also
>> have set *uid to 0, or changed a sensitive PF_ flag, or a number of other
>> things...
>>
>> Furthermore, think about it: there's literally an infinite amount of corrupted
>> task states that could be a security problem and that could be checked after every
>> system call. Do we want to check every one of them?
>
> Ok, I'm all for not checking lots of stuff all the time, just to protect
> from crappy drivers that.  Especially as we _can_ audit and run checks
> on the source code for them in the kernel tree.
>
> But, and here's the problem, outside of the desktop/enterprise world,
> there are a ton of out-of-tree code that is crap.  The number of
> security/bug fixes and kernel crashes for out-of-tree code in systems
> like Android phones is just so high it's laughable.
>
> When you have a device that is running 3.2 million lines of kernel code,
> yet the diffstat of the tree compared to mainline adds 3 million lines
> of code, there is bound to be a ton of issues/problems there.
>
> So this is an entirely different thing we need to try to protect
> ourselves from.  A long time ago I laughed when I saw that Microsoft had
> to do lots of "hardening" of their kernel to protect themselves from
> crappy drivers, as I knew we didn't have to do that because we had the
> source for them and could fix the root issues.  But that has changed and
> now we don't all have that option.  That code is out-of-tree because the
> vendor doesn't care, and doesn't want to take any time at all to do
> anything resembling a real code review[1].

That's a big part of why I thought would be useful. I am less worried
about edge cases upstream right now than forks with custom codes not
using set_fs correctly.

>
> So, how about options like the ones being proposed here, go behind a new
> config option:
>         CONFIG_PROTECT_FROM_CRAPPY_DRIVERS
> that device owners can enable if they do not trust their vendor-provided
> code (hint, I sure don't.)  That way the "normal" path that all of us
> are used to running will be fine, but if you want to take the speed hit
> to try to protect yourself, then you can do that as well.

Maybe another name but why not.

>
> Anyway, just an idea...
>
> thanks,
>
> greg k-h
>
> [1] I am working really hard with lots of vendors to try to fix their
>     broken development model, but that is going to take years to resolve
>     as their device pipelines are years long, and changing their
>     mindsets takes a long time...



-- 
Thomas