powerpc/lib/sstep: Don't use __{get/put}_user() on kernel addresses

powerpc/lib/sstep: Don't use __{get/put}_user() on kernel addresses

In the old days, when we didn't have kernel userspace access
protection and had set_fs(), it was wise to use __get_user()
and friends to read kernel memory.

Nowadays, get_user() and put_user() are granting userspace access and
are exclusively for userspace access.

Convert single step emulation functions to user_access_begin() and
friends and use unsafe_get_user() and unsafe_put_user().

When addressing kernel addresses, there is no need to open userspace
access. And for book3s/32 it is particularly important to no try and
open userspace access on kernel address, because that would break the
content of kernel space segment registers. No guard has been put
against that risk in order to avoid degrading performance.

copy_from_kernel_nofault() and copy_to_kernel_nofault() should
be used but they are out-of-line functions which would degrade
performance. Those two functions are making use of
__get_kernel_nofault() and __put_kernel_nofault() macros.
Those two macros are just wrappers behind __get_user_size_goto() and
__put_user_size_goto().

unsafe_get_user() and unsafe_put_user() are also wrappers of
__get_user_size_goto() and __put_user_size_goto(). Use them to
access kernel space. That allows refactoring userspace and
kernelspace access.

Reported-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Depends-on: 4fe5cda9f89d ("powerpc/uaccess: Implement user_read_access_begin and user_write_access_begin")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/22831c9d17f948680a12c5292e7627288b15f713.1631817805.git.christophe.leroy@csgroup.eu