lkml.org 
[lkml]   [2018]   [Jan]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 06/19] asm-generic/barrier: mask speculative execution flows
On Thu, Jan 11, 2018 at 04:46:56PM -0800, Dan Williams wrote:
> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
> new file mode 100644
> index 000000000000..5c66fc30f919
> --- /dev/null
> +++ b/include/linux/nospec.h
> @@ -0,0 +1,71 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// Copyright(c) 2018 Intel Corporation. All rights reserved.
> +
> +#ifndef __NOSPEC_H__
> +#define __NOSPEC_H__
> +
> +#include <linux/jump_label.h>
> +#include <asm/barrier.h>
> +
> +#ifndef array_ptr_mask
> +#define array_ptr_mask(idx, sz) \
> +({ \
> + unsigned long mask; \
> + unsigned long _i = (idx); \
> + unsigned long _s = (sz); \
> + \
> + mask = ~(long)(_i | (_s - 1 - _i)) >> (BITS_PER_LONG - 1); \
> + mask; \
> +})
> +#endif
> +
> +/**
> + * __array_ptr - Generate a pointer to an array element, ensuring
> + * the pointer is bounded under speculation to NULL.
> + *
> + * @base: the base of the array
> + * @idx: the index of the element, must be less than LONG_MAX
> + * @sz: the number of elements in the array, must be less than LONG_MAX
> + *
> + * If @idx falls in the interval [0, @sz), returns the pointer to
> + * @arr[@idx], otherwise returns NULL.
> + */
> +#define __array_ptr(base, idx, sz) \
> +({ \
> + union { typeof(*(base)) *_ptr; unsigned long _bit; } __u; \
> + typeof(*(base)) *_arr = (base); \
> + unsigned long _i = (idx); \
> + unsigned long _mask = array_ptr_mask(_i, (sz)); \
> + \
> + __u._ptr = _arr + (_i & _mask); \
> + __u._bit &= _mask; \
> + __u._ptr; \
> +})
> +
> +#ifdef CONFIG_SPECTRE1_IFENCE
> +DECLARE_STATIC_KEY_TRUE(nospec_key);
> +#else
> +DECLARE_STATIC_KEY_FALSE(nospec_key);
> +#endif
> +
> +#ifdef ifence_array_ptr
> +/*
> + * The expectation is that no compiler or cpu will mishandle __array_ptr
> + * leading to problematic speculative execution. Bypass the ifence
> + * based implementation by default.
> + */
> +#define array_ptr(base, idx, sz) \
> +({ \
> + typeof(*(base)) *__ret; \
> + \
> + if (static_branch_unlikely(&nospec_key)) \
> + __ret = ifence_array_ptr(base, idx, sz); \
> + else \
> + __ret = __array_ptr(base, idx, sz); \
> + __ret; \
> +})


So I think this wants:

#ifndef HAVE_JUMP_LABEL
#error Compiler lacks asm-goto, can generate unsafe code
#endif

Suppose the generic array_ptr_mask() is unsafe on some arch and they
only implement ifence_array_ptr() and they compile without asm-goto,
then the above reverts to a dynamic condition, which can be speculated.
If we then speculate into the 'bad' __array_ptr we're screwed.

> +#else
> +#define array_ptr __array_ptr
> +#endif
> +
> +#endif /* __NOSPEC_H__ */


In general I think I would write all this in a form like:

#define __array_ptr(base, idx, sz) \
({ \
union { typeof(*(base)) *_ptr; unsigned long _bit; } __u; \
typeof(*(base)) *_arr = (base); \
unsigned long _i = (idx); \
unsigned long _mask = array_ptr_mask(_i, (sz)); \
\
__u._ptr = _arr + (_i & _mask); \
__u._bit &= _mask; \
__u._ptr; \
})

#if defined(array_ptr_mask) && defined(ifence_array_ptr)

#ifndef HAVE_JUMP_LABEL
#error Compiler lacks asm-goto, can generate unsafe code
#endif

#define array_ptr(base, idx, sz) \
({ \
typeof(*(base)) *__ret; \
\
if (static_branch_unlikely(&nospec_key)) \
__ret = ifence_array_ptr(base, idx, sz); \
else \
__ret = __array_ptr(base, idx, sz); \
__ret; \
})

#elif defined(array_ptr_mask)

#define array_ptr(base, idx, sz) __array_ptr(base, idx, sz)

#elif defined(ifence_array_ptr)

#define array_ptr(base, idx, sz) ifence_array_ptr(base, idx, sz)

#else

/* XXX we want a suitable warning here ? */

#define array_ptr(base, idx, sz) (idx < sz ? base + idx : NULL)

#endif

and stick the generic array_ptr_mask into asm-generic/nospec.h or
something.

Then the static key stuff is limited to architectures that define _both_
array_ptr_mask and ifence_array_ptr.

\
 
 \ /
  Last update: 2018-01-14 23:25    [W:0.102 / U:3.768 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site