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

Ingo Molnar
2017-05-08T07:33:52Z

(added more Cc:s)

* Thomas Garnier <thgarnie@google.com> wrote:

> On Fri, Apr 28, 2017 at 8:32 AM, Thomas Garnier <thgarnie@google.com> wrote:
> > Ensure that a syscall does not return to user-mode with a kernel address
> > limit. If that happens, a process can corrupt kernel-mode memory and
> > elevate privileges [1].
> >
> > The CONFIG_ADDR_LIMIT_CHECK option disables the generic check so each
> > architecture can create optimized versions. This option is enabled by
> > default on s390 because a similar feature already exists.
> >
> > [1] https://bugs.chromium.org/p/project-zero/issues/detail?id=990
> >
> > Signed-off-by: Thomas Garnier <thgarnie@google.com>
> > Tested-by: Kees Cook <keescook@chromium.org>
>
> Ingo: Do you want to take the set?

Yeah, so now I'm questioning the whole premise of the feature, sorry :-/

A big disavantage is that the "security check" will add 2-5 instructions to the
system call fast path. Every one of them, and essentially forever. Just to handle
a CVE that was caused by a buggy touch-screen driver helper function leaking
KERNEL_DS and which was fixed long ago ...

And yes, I realize that there were other such bugs and that such bugs might occur
in the future - but why not push the overhead of the security check to the kernel
build phase? I.e. I'm wondering how well we could do static analysis during kernel
build - would a limited mode of Sparse be good enough for that? Or we could add a
new static checker to tools/, built from first principles and used primarily for
extended syntactical checking.

For example I'd consider it a good practice to mandate that if a kernel function
sets KERNEL_DS then it must restore it as well. Any function that does not do
that, or is too complex for the static analysis to prove correctness for sure
should be considered buggy!

Are there any common kernel APIs outside set_fs() that set KERNEL_DS
intentionally? The overwhelming pattern ought to be:

orig_fs = get_fs();
set_fs(KERNEL_DS);
...
set_fs(orig_fs);

... and even a relatively simple static analysis tool ought to be able to see
through that.

I'd even suggest we do it not like Sparse builds are done today, but in a more
integrated fashion: do static analysis as part of a typical kernel defconfig build
and not tolerate warnings but go for a 'zero warnings' policy like Linus uses for
modconfig builds.

_That_ solution I'd feel very, very good about - it would be so much better than
any runtime checks...

Not to mention that such an integrated static analysis facility would allow many
other things to be checked during build time, which we couldn't possibly check
runtime.

Thanks,

Ingo