Re: [PATCH v6 02/13] array_index_nospec: sanitize speculative array de-references

[Christian Borntraeger <borntraeger@de.ibm.com>]

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@linux-foundation.org>
> Co-developed-by: Alexei Starovoitov <ast@kernel.org>
> Suggested-by: Cyril Novikov <cnovikov@lynx.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: x86@kernel.org
> Signed-off-by: Dan Williams <dan.j.williams@intel.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 */
>