powerpc/mm: Fix lockup on kernel exec fault

In the Linux kernel, the following vulnerability has been resolved:

powerpc/mm: Fix lockup on kernel exec fault

The powerpc kernel is not prepared to handle exec faults from kernel.

Especially, the function is_exec_fault() will return 'false' when an

exec fault is taken by kernel, because the check is based on reading

current->thread.regs->trap which contains the trap from user.

For instance, when provoking a LKDTM EXEC_USERSPACE test,

current->thread.regs->trap is set to SYSCALL trap (0xc00), and

the fault taken by the kernel is not seen as an exec fault by

set_access_flags_filter().

Commit d7df2443cd5f ("powerpc/mm: Fix spurious segfaults on radix

with autonuma") made it clear and handled it properly. But later on

commit d3ca587404b3 ("powerpc/mm: Fix reporting of kernel execute

faults") removed that handling, introducing test based on error_code.

And here is the problem, because on the 603 all upper bits of SRR1

get cleared when the TLB instruction miss handler bails out to ISI.

Until commit cbd7e6ca0210 ("powerpc/fault: Avoid heavy

search_exception_tables() verification"), an exec fault from kernel

at a userspace address was indirectly caught by the lack of entry for

that address in the exception tables. But after that commit the

kernel mainly relies on KUAP or on core mm handling to catch wrong

user accesses. Here the access is not wrong, so mm handles it.

It is a minor fault because PAGE_EXEC is not set,

set_access_flags_filter() should set PAGE_EXEC and voila.

But as is_exec_fault() returns false as explained in the beginning,

set_access_flags_filter() bails out without setting PAGE_EXEC flag,

which leads to a forever minor exec fault.

As the kernel is not prepared to handle such exec faults, the thing to

do is to fire in bad_kernel_fault() for any exec fault taken by the

kernel, as it was prior to commit d3ca587404b3.