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

[Andy Lutomirski]

On Wed, May 10, 2017 at 1:14 AM, Christoph Hellwig <hch at infradead.org> wrote:
> On Wed, May 10, 2017 at 09:08:41AM +0100, Al Viro wrote:
>> On Wed, May 10, 2017 at 09:37:04AM +0200, Arnd Bergmann wrote:
>>
>> > > How about trying to remove all of them?  If we could actually get rid
>> > > of all of them, we could drop the arch support, and we'd get faster,
>> > > simpler, shorter uaccess code throughout the kernel.
>>
>> BTW, not all get_user() under KERNEL_DS are plain loads.  There is an
>> exception - probe_kernel_read().
>
> And various calls that looks like opencoded versions, e.g. drivers/dio
> or the ELF loader.
>
> But in the long run we'll just need a separate primitive for that,
> but that can wait until the set_fs calls outside the core code are
> gone.

I suspect that, on most arches, the primitive is called
__copy_from_user().  We could make the generic code do that except
where overridden.