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

Dan Williams
2018-01-10T02:22:10Z

On Tue, Jan 9, 2018 at 5:57 PM, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
> On Tue, Jan 09, 2018 at 04:48:24PM -0800, Dan Williams wrote:
>>
>> #define __nospec_array_ptr(base, idx, sz) \
>> ({ \
>> union { typeof(&base[0]) _ptr; unsigned long _bit; } __u; \
>> unsigned long _i = (idx); \
>> unsigned long _s = (sz); \
>> unsigned long _v = (long)(_i | _s - 1 - _i) \
>> >> BITS_PER_LONG - 1; \
>> unsigned long _mask = _v * ~0UL; \
>> OPTIMIZER_HIDE_VAR(_mask); \
>> __u._ptr = &base[_i & _mask]; \
>> __u._bit &= _mask; \
>> __u._ptr; \
>> })
>
> _v * ~0UL doesn't seem right and non intuitive.
> What's wrong with:
> unsigned long _mask = ~(long)(_i | _s - 1 - _i) >> BITS_PER_LONG - 1;

Yeah, I noticed it was ok immediately after I sent that.

> and why OPTIMIZER_HIDE_VAR ?

It was in Linus' original. but that was when it had the ternary
conditional, I'll drop it. It does not change the generated assembly.

> Could you remove '&' ?

Yes, that should be __u.ptr = base + (i & _mask)

> since in doesn't work for:
> struct {
> int fd[4];
> ...
> } *fdt;
> it cannot be used as array_acces(fdt->fd, ...);
>
> Could you please drop nospec_ prefix since it is misleading ?

When you came up with that tweak you noted:

"The following:
[..]
is generic and no speculative flows."

> This macro doesn't prevent speculation.

It masks dangerous speculation. At least, I read nospec as "No
Spectre" and it is a prefix used in the Spectre-v2 patches.

I also want to include the option, with a static branch, to switch it
to the hard "no speculation" version with an ifence if worse comes to
worse and we find a compiler / cpu where it doesn't work. The default
will be the fast and practical implementation.

> I think array_access() was the best name so far.

For other usages I need the pointer to the array element, also
array_access() by itself is unsuitable for __fcheck_files because we
still need rcu_dereference_raw() on the element de-reference. So, I
think it's better to get a sanitized array element pointer which can
be used with rcu, READ_ONCE(), etc... directly rather than try to do
the access in the same macro.