4.9-stable review patch. If anyone has any objections, please let me know.
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From: Dan Williams dan.j.williams@intel.com
(cherry picked from commit f3804203306e098dae9ca51540fcd5eb700d7f40)
array_index_nospec() is proposed as a generic mechanism to mitigate against Spectre-variant-1 attacks, i.e. an attack that bypasses boundary checks via speculative execution. The array_index_nospec() implementation is expected to be safe for current generation CPUs across multiple architectures (ARM, x86).
Based on an original implementation by Linus Torvalds, tweaked to remove speculative flows by Alexei Starovoitov, and tweaked again by Linus to introduce an x86 assembly implementation for the mask generation.
Co-developed-by: Linus Torvalds torvalds@linux-foundation.org Co-developed-by: Alexei Starovoitov ast@kernel.org Suggested-by: Cyril Novikov cnovikov@lynx.com Signed-off-by: Dan Williams dan.j.williams@intel.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Cc: linux-arch@vger.kernel.org Cc: kernel-hardening@lists.openwall.com Cc: Peter Zijlstra peterz@infradead.org Cc: Catalin Marinas catalin.marinas@arm.com Cc: Will Deacon will.deacon@arm.com Cc: Russell King linux@armlinux.org.uk Cc: gregkh@linuxfoundation.org Cc: torvalds@linux-foundation.org Cc: alan@linux.intel.com Link: https://lkml.kernel.org/r/151727414229.33451.18411580953862676575.stgit@dwil... Signed-off-by: David Woodhouse dwmw@amazon.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/nospec.h | 72 +++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 72 insertions(+) create mode 100644 include/linux/nospec.h
--- /dev/null +++ b/include/linux/nospec.h @@ -0,0 +1,72 @@ +// SPDX-License-Identifier: GPL-2.0 +// Copyright(c) 2018 Linus Torvalds. All rights reserved. +// Copyright(c) 2018 Alexei Starovoitov. All rights reserved. +// Copyright(c) 2018 Intel Corporation. All rights reserved. + +#ifndef _LINUX_NOSPEC_H +#define _LINUX_NOSPEC_H + +/** + * array_index_mask_nospec() - generate a ~0 mask when index < size, 0 otherwise + * @index: array element index + * @size: number of elements in array + * + * When @index is out of bounds (@index >= @size), the sign bit will be + * set. Extend the sign bit to all bits and invert, giving a result of + * zero for an out of bounds index, or ~0 if within bounds [0, @size). + */ +#ifndef array_index_mask_nospec +static inline unsigned long array_index_mask_nospec(unsigned long index, + unsigned long size) +{ + /* + * Warn developers about inappropriate array_index_nospec() usage. + * + * Even if the CPU speculates past the WARN_ONCE branch, the + * sign bit of @index is taken into account when generating the + * mask. + * + * This warning is compiled out when the compiler can infer that + * @index and @size are less than LONG_MAX. + */ + if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX, + "array_index_nospec() limited to range of [0, LONG_MAX]\n")) + return 0; + + /* + * Always calculate and emit the mask even if the compiler + * thinks the mask is not needed. The compiler does not take + * into account the value of @index under speculation. + */ + OPTIMIZER_HIDE_VAR(index); + return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1); +} +#endif + +/* + * array_index_nospec - sanitize an array index after a bounds check + * + * For a code sequence like: + * + * if (index < size) { + * index = array_index_nospec(index, size); + * val = array[index]; + * } + * + * ...if the CPU speculates past the bounds check then + * array_index_nospec() will clamp the index within the range of [0, + * size). + */ +#define array_index_nospec(index, size) \ +({ \ + typeof(index) _i = (index); \ + typeof(size) _s = (size); \ + unsigned long _mask = array_index_mask_nospec(_i, _s); \ + \ + BUILD_BUG_ON(sizeof(_i) > sizeof(long)); \ + BUILD_BUG_ON(sizeof(_s) > sizeof(long)); \ + \ + _i &= _mask; \ + _i; \ +}) +#endif /* _LINUX_NOSPEC_H */