93 lines
2.9 KiB
ReStructuredText
93 lines
2.9 KiB
ReStructuredText
===========
|
|
Speculation
|
|
===========
|
|
|
|
This document explains potential effects of speculation, and how undesirable
|
|
effects can be mitigated portably using common APIs.
|
|
|
|
------------------------------------------------------------------------------
|
|
|
|
To improve performance and minimize average latencies, many contemporary CPUs
|
|
employ speculative execution techniques such as branch prediction, performing
|
|
work which may be discarded at a later stage.
|
|
|
|
Typically speculative execution cannot be observed from architectural state,
|
|
such as the contents of registers. However, in some cases it is possible to
|
|
observe its impact on microarchitectural state, such as the presence or
|
|
absence of data in caches. Such state may form side-channels which can be
|
|
observed to extract secret information.
|
|
|
|
For example, in the presence of branch prediction, it is possible for bounds
|
|
checks to be ignored by code which is speculatively executed. Consider the
|
|
following code::
|
|
|
|
int load_array(int *array, unsigned int index)
|
|
{
|
|
if (index >= MAX_ARRAY_ELEMS)
|
|
return 0;
|
|
else
|
|
return array[index];
|
|
}
|
|
|
|
Which, on arm64, may be compiled to an assembly sequence such as::
|
|
|
|
CMP <index>, #MAX_ARRAY_ELEMS
|
|
B.LT less
|
|
MOV <returnval>, #0
|
|
RET
|
|
less:
|
|
LDR <returnval>, [<array>, <index>]
|
|
RET
|
|
|
|
It is possible that a CPU mis-predicts the conditional branch, and
|
|
speculatively loads array[index], even if index >= MAX_ARRAY_ELEMS. This
|
|
value will subsequently be discarded, but the speculated load may affect
|
|
microarchitectural state which can be subsequently measured.
|
|
|
|
More complex sequences involving multiple dependent memory accesses may
|
|
result in sensitive information being leaked. Consider the following
|
|
code, building on the prior example::
|
|
|
|
int load_dependent_arrays(int *arr1, int *arr2, int index)
|
|
{
|
|
int val1, val2,
|
|
|
|
val1 = load_array(arr1, index);
|
|
val2 = load_array(arr2, val1);
|
|
|
|
return val2;
|
|
}
|
|
|
|
Under speculation, the first call to load_array() may return the value
|
|
of an out-of-bounds address, while the second call will influence
|
|
microarchitectural state dependent on this value. This may provide an
|
|
arbitrary read primitive.
|
|
|
|
====================================
|
|
Mitigating speculation side-channels
|
|
====================================
|
|
|
|
The kernel provides a generic API to ensure that bounds checks are
|
|
respected even under speculation. Architectures which are affected by
|
|
speculation-based side-channels are expected to implement these
|
|
primitives.
|
|
|
|
The array_index_nospec() helper in <linux/nospec.h> can be used to
|
|
prevent information from being leaked via side-channels.
|
|
|
|
A call to array_index_nospec(index, size) returns a sanitized index
|
|
value that is bounded to [0, size) even under cpu speculation
|
|
conditions.
|
|
|
|
This can be used to protect the earlier load_array() example::
|
|
|
|
int load_array(int *array, unsigned int index)
|
|
{
|
|
if (index >= MAX_ARRAY_ELEMS)
|
|
return 0;
|
|
else {
|
|
index = array_index_nospec(index, MAX_ARRAY_ELEMS);
|
|
return array[index];
|
|
}
|
|
}
|