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

[Brian Gerst]

On Tue, May 9, 2017 at 9:00 AM, Andy Lutomirski <luto at kernel.org> wrote:
> On Tue, May 9, 2017 at 1:56 AM, Christoph Hellwig <hch at 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