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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
|
