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Diffstat (limited to 'common/recipes-kernel/linux/linux-yocto-4.9.21/0024-Documentation-Document-array_index_nospec.patch')
| -rw-r--r-- | common/recipes-kernel/linux/linux-yocto-4.9.21/0024-Documentation-Document-array_index_nospec.patch | 128 |
1 files changed, 128 insertions, 0 deletions
diff --git a/common/recipes-kernel/linux/linux-yocto-4.9.21/0024-Documentation-Document-array_index_nospec.patch b/common/recipes-kernel/linux/linux-yocto-4.9.21/0024-Documentation-Document-array_index_nospec.patch new file mode 100644 index 00000000..e63a87b0 --- /dev/null +++ b/common/recipes-kernel/linux/linux-yocto-4.9.21/0024-Documentation-Document-array_index_nospec.patch @@ -0,0 +1,128 @@ +From be059366798cbe4d7f4e9d86232e17b2368154ce Mon Sep 17 00:00:00 2001 +From: Mark Rutland <mark.rutland@arm.com> +Date: Mon, 29 Jan 2018 17:02:16 -0800 +Subject: [PATCH 24/42] Documentation: Document array_index_nospec + +(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@dwillia2-desk3.amr.corp.intel.com +Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> +Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> +--- + 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]; ++ } ++ } +-- +2.7.4 + |