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author | Jens Axboe <axboe@kernel.dk> | 2018-02-05 12:55:38 -0700 |
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committer | Jens Axboe <axboe@kernel.dk> | 2018-02-05 12:55:38 -0700 |
commit | 9e05c864993c5442227f83ae1694a737d7a102ed (patch) | |
tree | e35b60bb3c0c179f147e9acaad5444f1e5d9117e /Documentation/speculation.txt | |
parent | 3c15f3f545afa320c5e3822825a9a53c664776b6 (diff) | |
parent | 35277995e17919ab838beae765f440674e8576eb (diff) |
Merge branch 'master' into test
* master: (688 commits)
dt-bindings: mailbox: qcom: Document the APCS clock binding
mailbox: qcom: Create APCS child device for clock controller
mailbox: qcom: Convert APCS IPC driver to use regmap
KVM/SVM: Allow direct access to MSR_IA32_SPEC_CTRL
KVM/VMX: Allow direct access to MSR_IA32_SPEC_CTRL
KVM/VMX: Emulate MSR_IA32_ARCH_CAPABILITIES
KVM/x86: Add IBPB support
KVM/x86: Update the reverse_cpuid list to include CPUID_7_EDX
pinctrl: remove include file from <linux/device.h>
firmware: dmi: handle missing DMI data gracefully
firmware: dmi_scan: Fix handling of empty DMI strings
firmware: dmi_scan: Drop dmi_initialized
firmware: dmi: Optimize dmi_matches
Revert "defer call to mem_cgroup_sk_alloc()"
soreuseport: fix mem leak in reuseport_add_sock()
net: qlge: use memmove instead of skb_copy_to_linear_data
net: qed: use correct strncpy() size
net: cxgb4: avoid memcpy beyond end of source buffer
cls_u32: add missing RCU annotation.
r8152: set rx mode early when linking on
...
Diffstat (limited to 'Documentation/speculation.txt')
-rw-r--r-- | Documentation/speculation.txt | 90 |
1 files changed, 90 insertions, 0 deletions
diff --git a/Documentation/speculation.txt b/Documentation/speculation.txt new file mode 100644 index 000000000000..e9e6cbae2841 --- /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]; + } + } |