1 ===========
2 Speculation
3 ===========
4
5 This document explains potential effects of speculation, and how undesirable
6 effects can be mitigated portably using common APIs.
7
8 ------------------------------------------------------------------------------
9
10 To improve performance and minimize average latencies, many contemporary CPUs
11 employ speculative execution techniques such as branch prediction, performing
12 work which may be discarded at a later stage.
13
14 Typically speculative execution cannot be observed from architectural state,
15 such as the contents of registers. However, in some cases it is possible to
16 observe its impact on microarchitectural state, such as the presence or
17 absence of data in caches. Such state may form side-channels which can be
18 observed to extract secret information.
19
20 For example, in the presence of branch prediction, it is possible for bounds
21 checks to be ignored by code which is speculatively executed. Consider the
22 following code::
23
24 int load_array(int *array, unsigned int index)
25 {
26 if (index >= MAX_ARRAY_ELEMS)
27 return 0;
28 else
29 return array[index];
30 }
31
32 Which, on arm64, may be compiled to an assembly sequence such as::
33
34 CMP <index>, #MAX_ARRAY_ELEMS
35 B.LT less
36 MOV <returnval>, #0
37 RET
38 less:
39 LDR <returnval>, [<array>, <index>]
40 RET
41
42 It is possible that a CPU mis-predicts the conditional branch, and
43 speculatively loads array[index], even if index >= MAX_ARRAY_ELEMS. This
44 value will subsequently be discarded, but the speculated load may affect
45 microarchitectural state which can be subsequently measured.
46
47 More complex sequences involving multiple dependent memory accesses may
48 result in sensitive information being leaked. Consider the following
49 code, building on the prior example::
50
51 int load_dependent_arrays(int *arr1, int *arr2, int index)
52 {
53 int val1, val2,
54
55 val1 = load_array(arr1, index);
56 val2 = load_array(arr2, val1);
57
58 return val2;
59 }
60
61 Under speculation, the first call to load_array() may return the value
62 of an out-of-bounds address, while the second call will influence
63 microarchitectural state dependent on this value. This may provide an
64 arbitrary read primitive.
65
66 ====================================
67 Mitigating speculation side-channels
68 ====================================
69
70 The kernel provides a generic API to ensure that bounds checks are
71 respected even under speculation. Architectures which are affected by
72 speculation-based side-channels are expected to implement these
73 primitives.
74
75 The array_index_nospec() helper in <linux/nospec.h> can be used to
76 prevent information from being leaked via side-channels.
77
78 A call to array_index_nospec(index, size) returns a sanitized index
79 value that is bounded to [0, size) even under cpu speculation
80 conditions.
81
82 This can be used to protect the earlier load_array() example::
83
84 int load_array(int *array, unsigned int index)
85 {
86 if (index >= MAX_ARRAY_ELEMS)
87 return 0;
88 else {
89 index = array_index_nospec(index, MAX_ARRAY_ELEMS);
90 return array[index];
91 }
92 }
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