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92 lines
2.9 KiB
92 lines
2.9 KiB
=========== |
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Speculation |
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=========== |
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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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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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