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Date: Fri, 16 Feb 2018 09:55:05 +0100
From: Christian Borntraeger <borntraeger@...ibm.com>
To: Dan Williams <dan.j.williams@...el.com>, tglx@...utronix.de,
mingo@...nel.org
Cc: linux-arch@...r.kernel.org, Cyril Novikov <cnovikov@...x.com>,
kernel-hardening@...ts.openwall.com,
Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>, x86@...nel.org,
Will Deacon <will.deacon@....com>,
Russell King <linux@...linux.org.uk>, linux-kernel@...r.kernel.org,
Ingo Molnar <mingo@...hat.com>, gregkh@...uxfoundation.org,
"H. Peter Anvin" <hpa@...or.com>, torvalds@...ux-foundation.org,
alan@...ux.intel.com
Subject: Re: [PATCH v6 02/13] array_index_nospec: sanitize speculative array
de-references
On 01/30/2018 02:02 AM, Dan Williams wrote:
> 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@...ux-foundation.org>
> Co-developed-by: Alexei Starovoitov <ast@...nel.org>
> Suggested-by: Cyril Novikov <cnovikov@...x.com>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
> include/linux/nospec.h | 72 ++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 72 insertions(+)
> create mode 100644 include/linux/nospec.h
>
> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
> new file mode 100644
> index 000000000000..cf7be4bbcf17
> --- /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
Hmmm, shouldn't we include asm/barrier.h here? Otherwise users might or might not
use the optimized variant depending on which headers are included by the users
of array_index_nospec.
> +
> +/**
> + * 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 */
>
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