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

Brian Gerst
2017-05-09T16:05:12Z

On Tue, May 9, 2017 at 9:00 AM, Andy Lutomirski <luto@kernel.org> wrote:
> On Tue, May 9, 2017 at 1:56 AM, Christoph Hellwig <hch@infradead.org> wrote:
>> On Tue, May 09, 2017 at 08:45:22AM +0200, Ingo Molnar wrote:
>>> We only have ~115 code blocks in the kernel that set/restore KERNEL_DS, it would
>>> be a pity to add a runtime check to every system call ...
>>
>> I think we should simply strive to remove all of them that aren't
>> in core scheduler / arch code. Basically evetyytime we do the
>>
>> oldfs = get_fs();
>> set_fs(KERNEL_DS);
>> ..
>> set_fs(oldfs);
>>
>> trick we're doing something wrong, and there should always be better
>> ways to archive it. E.g. using iov_iter with a ITER_KVEC type
>> consistently would already remove most of them.
>
> 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.
>
> The ones in kernel/compat.c are generally garbage. They should be
> using compat_alloc_user_space(). Ditto for kernel/power/user.c.

compat_alloc_user_space() is a hack that should go away too. It ends
up copying the data three times.

The more efficient solution to this is to have a core syscall function
that only accesses kernel memory, and then have two front-end
functions (native and compat) that do the actual reads and writes to
userspace, with conversion in the compat case.

--
Brian Gerst