This document explains potential effects of speculation, and how undesirable
  effects can be mitigated portably using common APIs.
  
  ===========
  Speculation
  ===========
  
  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];
  		}
  	}