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