powerpc: Fix kexec failure on book3s/32

powerpc: Fix kexec failure on book3s/32

In the old days, _PAGE_EXEC didn't exist on 6xx aka book3s/32.
Therefore, allthough __mapin_ram_chunk() was already mapping kernel
text with PAGE_KERNEL_TEXT and the rest with PAGE_KERNEL, the entire
memory was executable. Part of the memory (first 512kbytes) was
mapped with BATs instead of page table, but it was also entirely
mapped as executable.

In commit 385e89d5b20f ("powerpc/mm: add exec protection on
powerpc 603"), we started adding exec protection to some 6xx, namely
the 603, for pages mapped via pagetables.

Then, in commit 63b2bc619565 ("powerpc/mm/32s: Use BATs for
STRICT_KERNEL_RWX"), the exec protection was extended to BAT mapped
memory, so that really only the kernel text could be executed.

The problem here is that kexec is based on copying some code into
upper part of memory then executing it from there in order to install
a fresh new kernel at its definitive location.

However, the code is position independant and first part of it is
just there to deactivate the MMU and jump to the second part. So it
is possible to run this first part inplace instead of running the
copy. Once the MMU is off, there is no protection anymore and the
second part of the code will just run as before.

Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Fixes: 63b2bc619565 ("powerpc/mm/32s: Use BATs for STRICT_KERNEL_RWX")
Cc: stable@vger.kernel.org # v5.1+
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>