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SubjectRe: [RFCv2 1/4] asm-generic/barrier: add generic nospec helpers
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On Fri, 2018-01-05 at 14:57 +0000, Mark Rutland wrote:
> Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
> memory accesses under a bounds check may be speculated even if the
> bounds check fails, providing a primitive for building a side channel.
>
> This patch adds helpers which can be used to inhibit the use of
> out-of-bounds pointers under speculation.
>
> A generic implementation is provided for compatibility, but does not
> guarantee safety under speculation. Architectures are expected to
> override these helpers as necessary.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> Cc: Daniel Willams <dan.j.williams@intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
> include/asm-generic/barrier.h | 68 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 68 insertions(+)
>
> Dan, I've reworked this so that nospec_ptr() can take an arch-specific barrier
> sequence. I believe that for x86 you just need to implement __nospec_barrier()
> as osb().
>
> Mark.
>
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index fe297b599b0a..91c3071f49e5 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -54,6 +54,74 @@
> #define read_barrier_depends() do { } while (0)
> #endif
>
> +/*
> + * Inhibit subsequent speculative memory accesses.
> + *
> + * Architectures with a suitable memory barrier should provide an
> + * implementation. This is non-portable, and generic code should use
> + * nospec_ptr().
> + */
> +#ifndef __nospec_barrier
> +#define __nospec_barrier() do { } while (0)
> +#endif
> +
> +/**
> + * nospec_ptr() - Ensure a pointer is bounded, even under speculation.
> + *
> + * @ptr: the pointer to test
> + * @lo: the lower valid bound for @ptr, inclusive
> + * @hi: the upper valid bound for @ptr, exclusive
> + *
> + * If @ptr falls in the interval [@lo, @i), returns @ptr, otherwise returns
> + * NULL.
> + *
> + * Architectures which do not provide __nospec_barrier() should override this
> + * to ensure that ptr falls in the [lo, hi) interval both under architectural
> + * execution and under speculation, preventing propagation of an out-of-bounds
> + * pointer to code which is speculatively executed.
> + */
> +#ifndef nospec_ptr
> +#define nospec_ptr(ptr, lo, hi) \
> +({ \
> + typeof (ptr) __ret; \
> + typeof (ptr) __ptr = (ptr); \
> + typeof (ptr) __lo = (lo); \
> + typeof (ptr) __hi = (hi); \
> + \
> + __ret = (__lo <= __ptr && __ptr < __hi) ? __ptr : NULL; \
> + \
> + __nospec_barrier(); \
> + \
> + __ret; \
> +})
> +#endif
> +
> +/**
> + * nospec_array_ptr - Generate a pointer to an array element, ensuring the
> + * pointer is bounded under speculation.
> + *
> + * @arr: the base of the array
> + * @idx: the index of the element
> + * @sz: the number of elements in the array
> + *
> + * If @idx falls in the interval [0, @sz), returns the pointer to @arr[@idx],
> + * otherwise returns NULL.
> + *
> + * This is a wrapper around nospec_ptr(), provided for convenience.
> + * Architectures should implement nospec_ptr() to ensure this is the case
> + * under speculation.
> + */
> +#define nospec_array_ptr(arr, idx, sz) \
> +({ \
> + typeof(*(arr)) *__arr = (arr); \
> + typeof(idx) __idx = (idx); \
> + typeof(sz) __sz = (sz); \
> + \
> + nospec_ptr(__arr + __idx, __arr, __arr + __sz); \
> +})
> +
> +#undef __nospec_barrier

I see:

kernel/bpf/arraymap.c: In function 'array_map_lookup_elem':
kernel/bpf/arraymap.c:124:2: error: implicit declaration of function '__nospec_barrier' [-Werror=implicit-function-declaration]
return nospec_ptr(ptr, array->value, high);
^
cc1: all warnings being treated as errors

when building using a 4.8ish gcc. Removing the above #undef avoids that
error. I don't get the build error on fedora with gcc7

> +
> #ifndef __smp_mb
> #define __smp_mb() mb()
> #endif
\
 
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