linux-stable/include/linux/nospec.h
Dave Hansen 74e19ef0ff uaccess: Add speculation barrier to copy_from_user()
The results of "access_ok()" can be mis-speculated.  The result is that
you can end speculatively:

	if (access_ok(from, size))
		// Right here

even for bad from/size combinations.  On first glance, it would be ideal
to just add a speculation barrier to "access_ok()" so that its results
can never be mis-speculated.

But there are lots of system calls just doing access_ok() via
"copy_to_user()" and friends (example: fstat() and friends).  Those are
generally not problematic because they do not _consume_ data from
userspace other than the pointer.  They are also very quick and common
system calls that should not be needlessly slowed down.

"copy_from_user()" on the other hand uses a user-controller pointer and
is frequently followed up with code that might affect caches.  Take
something like this:

	if (!copy_from_user(&kernelvar, uptr, size))
		do_something_with(kernelvar);

If userspace passes in an evil 'uptr' that *actually* points to a kernel
addresses, and then do_something_with() has cache (or other)
side-effects, it could allow userspace to infer kernel data values.

Add a barrier to the common copy_from_user() code to prevent
mis-speculated values which happen after the copy.

Also add a stub for architectures that do not define barrier_nospec().
This makes the macro usable in generic code.

Since the barrier is now usable in generic code, the x86 #ifdef in the
BPF code can also go away.

Reported-by: Jordy Zomer <jordyzomer@google.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>   # BPF bits
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2023-02-21 14:45:22 -08:00

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2.2 KiB
C

// 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
#include <linux/compiler.h>
#include <asm/barrier.h>
struct task_struct;
#ifndef barrier_nospec
# define barrier_nospec() do { } while (0)
#endif
/**
* 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)
{
/*
* 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)); \
\
(typeof(_i)) (_i & _mask); \
})
/* Speculation control prctl */
int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which);
int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
unsigned long ctrl);
/* Speculation control for seccomp enforced mitigation */
void arch_seccomp_spec_mitigate(struct task_struct *task);
#endif /* _LINUX_NOSPEC_H */