Re: [PATCH 16/18] net: mpls: prevent bounds-check bypass via speculative execution

Dan Williams
2018-01-09T03:42:16Z

On Mon, Jan 8, 2018 at 7:11 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
> Dan Williams <dan.j.williams@intel.com> writes:
>
>> Static analysis reports that 'index' may be a user controlled value that
>> is used as a data dependency reading 'rt' from the 'platform_label'
>> array. In order to avoid potential leaks of kernel memory values, block
>> speculative execution of the instruction stream that could issue further
>> reads based on an invalid 'rt' value.
>
>
> In detail.
> a) This code is fast path packet forwarding code. Introducing an
> unconditional pipeline stall is not ok.
>
> AKA either there is no speculation and so this is invulnerable
> or there is speculation and you are creating an unconditional
> pipeline stall here.
>
> My back of the napkin caluculations say that a pipeline stall
> is about 20 cycles. Which is about the same length of time
> as a modern cache miss.
>
> On a good day this code will perform with 0 cache misses. On a less
> good day 1 cache miss. Which means you are quite possibly doubling
> the runtime of mpls_forward.
>
> b) The array is dynamically allocated which should provide some
> protection, as it will be more difficult to predict the address
> of the array which is needed to craft an malicious userspace value.
>
> c) The code can be trivially modified to say:
>
> static struct mpls_route *mpls_route_input_rcu(struct net *net, unsigned index)
> {
> struct mpls_route *rt = NULL;
>
> if (index < net->mpls.platform_labels) {
> struct mpls_route __rcu **platform_label =
> rcu_dereference(net->mpls.platform_label);
> rt = rcu_dereference(platform_label[index & ((1 << 20) - 1)]);
> }
> return rt;
> }
>
> AKA a static mask will ensure that there is not a primitive that can be
> used to access all of memory. That is max a 1 cycle slowdown in the
> code, which is a much better trade off.
>
> d) If we care more it is straight forward to modify
> resize_platform_label_table() to ensure that the size of the array
> is always a power of 2.
>
> e) The fact that a pointer is returned from the array and it is treated
> like a pointer would seem to provide a defense against the
> exfiltration technique of using the value read as an index into
> a small array, that user space code can probe aliased cached
> lines of, to see which value was dereferenced.
>
>
> So to this patch in particular.
> Nacked-by: "Eric W. Biederman" <ebiederm@xmission.com>
>
> This code path will be difficult to exploit. This change messes with
> performance. There are ways to make this code path useless while
> preserving the performance of the code.
>

Thanks, Eric understood. The discussion over the weekend came to the
conclusion that using a mask will be the default approach. The
nospec_array_ptr() will be defined to something similar to the
following, originally from Linus and tweaked by Alexei and I:

#define __nospec_array_ptr(base, idx, sz) \
({ \
union { typeof(&base[0]) _ptr; unsigned long _bit; } __u; \
unsigned long _i = (idx); \
unsigned long _m = (max); \
unsigned long _mask = ~(long)(_m - 1 - _i) >> BITS_PER_LONG - 1;\
OPTIMIZER_HIDE_VAR(_mask); \
__u._ptr = &base[_i & _mask]; \
__u._bit &= _mask; \
__u._ptr; \
})

Does that address your performance concerns?