On Fri, Jul 31, 2015 at 1:59 PM, <josh@xxxxxxxxxxxxxxxx> wrote:
> On Fri, Jul 31, 2015 at 11:56:06AM -0700, Kees Cook wrote:
>> On Thu, Jul 30, 2015 at 6:02 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
>> > On Thu, Jul 30, 2015 at 01:03:43PM -0700, Kees Cook wrote:
>> >> On Thu, Jul 30, 2015 at 12:04 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
>> >> > On Thu, Jul 30, 2015 at 11:21:54AM -0700, Kees Cook wrote:
>> >> >> I like this, it's a good description of both options. I'm still biased
>> >> >> about the approach: I prefer flags, since pointers to user structures
>> >> >> complicate syscall filtering. ;)
>> >> >
>> >> > Seems like we should do two things to make that easier:
>> >> >
>> >> > 1) Create a standardized kernel mechanism for parameter-struct handling,
>> >> > implementing the recommendations mentioned here.
>> >>
>> >> It's been suggested in the past that nlmsg is appropriate for such a
>> >> thing, but I remain suspicious. :)
>> >
>> > Likewise. :)
>> >
>> >> > 2) Integrate into that mechanism a way to filter the resulting parameter
>> >> > struct with BPF *after* it has been copied to kernel space (and thus
>> >> > can no longer be tampered with).
>> >>
>> >> Yeah, this is a irritating part: the structures operated on are copied
>> >> from userspace adhoc in each syscall. Doing argument checking would
>> >> mean double copies initially, and perhaps teaching syscalls about
>> >> optional "already copied" arguments or something as an optimization.
>> >
>> > No, double copies can't work for security reasons. Because otherwise
>> > you could race the kernel from another thread, substituting different
>> > values after the check and before the use.
>>
>> Right, the double copy method would require setting up a per-thread
>> userspace memory mapping that was read-only from userspace but
>> writable from kernel space.
>
> Which seems like a lot more trouble than just copying it once.
>
>> > I think the right API looks *roughly* like this:
>> >
>> > int _copy_param_struct(size_t kernel_len, void *kernel_struct, size_t user_len, void __user *user_struct)
>> > {
>> > if (user_len > kernel_len)
>> > return -EINVAL;
>> > if (user_len && copy_from_user(kernel_struct, user_struct, user_len))
>> > return -EFAULT;
>> > if (user_len < kernel_len)
>> > memset(kernel_struct + user_len, 0, kernel_len - user_len);
>> > return 0;
>> > }
>> >
>> > #define copy_param_struct(kernel_struct, user_len, user_struct) _copy_param_struct( \
>> > sizeof(*kernel_struct) + BUILD_BUG_ON_ZERO(!__same_type(*kernel_struct, *user_struct)), \
>> > kernel_struct, user_len, user_struct)
>> >
>> >
>> > Then the syscall looks like this:
>> >
>> > SYSCALL_DEFINEn(xyzzy, ..., ..., size_t user_params_len, struct xyzzy_params __user *user_params)
>> > {
>> > int ret;
>> > struct xyzzy_params params;
>> >
>> > ret = copy_param_struct(¶ms, user_params_len, user_params);
>> > if (ret)
>> > return ret;
>> > ...
>> >
>> >
>> > And you could then hook copy_params_struct to add arbitrary additional
>> > syscall parameter validation. Bonus if there's some way to make the
>> > copy and validation occur before the syscall is ever invoked, rather
>> > than inside the syscall, but that would require adding fancier syscall
>> > definition mechanisms that autogenerate such code.
>>
>> The trouble is that the hook for the syscall (both seccomp and ptrace)
>> happens before the sys_* function executes. So the param extract
>> suddenly becomes optional. As in, did ptrace/seccomp already extract
>> the args? If so, use that copy, else copy them out myself now that I
>> need them, etc.
>>
>> It's entirely doable, but it's going to require some careful design.
>
> Agreed. I think the proposal above would be a net improvement, but
> ideally you'd want something that's annotated and generates automatic
> marshalling code.
>
I assume this is idle musing. If, however, we were to actually do
this, I'd suggest we seriously consider speaking the Cap'n Proto
serialization format. It's quite nice, it encodes and decodes *very*
quickly and, unlike TLV schemes, you don't have to read it in order,
making the read-side code less awkward.
(I wouldn't suggest that we use the official C++11 library, but the
format might be a good idea.)
--Andy
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