Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks

[Laura Abbott <labbott@redhat.com>]

On 10/26/2017 02:09 AM, Mark Rutland wrote:
> Hi,
> 
> In Prague, Kees mentioned that it would be nice to have a mechanism to
> catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2]
> issue with unsafe_put_user() in waitid().
> 
> These patches allow an optional access_ok() check to be dropped in
> arm64's __{get,put}_user() primitives. These will then BUG() if a bad
> user pointer is passed (which should only happen in the absence of an
> earlier access_ok() check).
> 
> The first patch rewrites the arm64 access_ok() check in C. This gives
> the compiler the visibility it needs to elide redundant access_ok()
> checks, so in the common case:
> 
>   get_user()
>     access_ok()
>     __get_user()
>       BUG_ON(!access_ok())
>       <uaccess asm>
> 
> ... the compiler can determine that the second access_ok() must return
> true, and can elide it along with the BUG_ON(), leaving:
> 
>   get_user()
>     access_ok()
>       __get_user()
>         <uaccess asm>
> 
> ... and thus this sanity check can have no cost in the common case.
> 
> The compiler doesn't always have the visibility to do this (e.g. if the
> two access_ok() checks are in different compilation units), but it seems
> to manage to do this most of the time -- In testing with v4.14-rc5
> defconfig this only increases the total Image size by 4KiB.
> 
> I had a go at turning this into a BUILD_BUG_ON(), to see if we could
> catch this issue at compile time. However, my GCC wasn't able to remove
> the BUILD_BUG() from some {get,put}_user cases. Maybe we can fix that,
> or maybe we can have some static analysis catch this at build time.
> 
> It's entirely possible that I've made some catastrophic mistake in these
> patches; I've only build-tested them so far, and I don't currently have
> access to hardware to test on.
> 
> I also haven't yet modified __copy_{to,from}_user and friends along
> similar lines, so this is incomplete. If there aren't any major
> objections to this approach, I can fold those in for the next spin.
> 
> Thanks,
> Mark.
> 
> [1] https://lwn.net/Articles/736348/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=96ca579a1ecc943b75beba58bebb0356f6cc4b51
> 
> 
> Mark Rutland (2):
>   arm64: write __range_ok() in C
>   arm64: allow paranoid __{get,put}user
> 
>  arch/arm64/Kconfig               |  9 +++++++++
>  arch/arm64/include/asm/uaccess.h | 27 +++++++++++++++++++--------
>  2 files changed, 28 insertions(+), 8 deletions(-)
> 

Turning on the option fails as soon as we hit userspace. On my buildroot
based environment I get the help text for ld.so (????) and then a message
about attempting to kill init. I get a crash in init on the Hikey Android
environment as well. It almost seems like the __range_ok re-write
is triggering an error but it only seems to happen when the option is
enabled even when I take out the BUG. I'll see if I can get more useful
information.

Thanks,
Laura