From: Mark Rutland mark.rutland@arm.com
(cherry picked from commit f84a56f73dddaeac1dba8045b007f742f61cd2da)
Document the rationale and usage of the new array_index_nospec() helper.
Signed-off-by: Mark Rutland mark.rutland@arm.com Signed-off-by: Will Deacon will.deacon@arm.com Signed-off-by: Dan Williams dan.j.williams@intel.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Kees Cook keescook@chromium.org Cc: linux-arch@vger.kernel.org Cc: Jonathan Corbet corbet@lwn.net Cc: Peter Zijlstra peterz@infradead.org Cc: gregkh@linuxfoundation.org Cc: kernel-hardening@lists.openwall.com Cc: torvalds@linux-foundation.org Cc: alan@linux.intel.com Link: https://lkml.kernel.org/r/151727413645.33451.15878817161436755393.stgit@dwil... Signed-off-by: David Woodhouse dwmw@amazon.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org [jwang: cherry pick to 4.4] Signed-off-by: Jack Wang jinpu.wang@profitbricks.com --- Documentation/speculation.txt | 90 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 90 insertions(+) create mode 100644 Documentation/speculation.txt
diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 0000000..e9e6cba --- /dev/null +++ b/Documentation/speculation.txt @@ -0,0 +1,90 @@ +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]; + } + }