powerpc/tm: Avoid machine crash on rt_sigreturn()

powerpc/tm: Avoid machine crash on rt_sigreturn()

There is a kernel crash that happens if rt_sigreturn() is called inside
a transactional block.

This crash happens if the kernel hits an in-kernel page fault when
accessing userspace memory, usually through copy_ckvsx_to_user(). A
major page fault calls might_sleep() function, which can cause a task
reschedule. A task reschedule (switch_to()) reclaim and recheckpoint
the TM states, but, in the signal return path, the checkpointed memory
was already reclaimed, thus the exception stack has MSR that points to
MSR[TS]=0.

When the code returns from might_sleep() and a task reschedule
happened, then this task is returned with the memory recheckpointed,
and CPU MSR[TS] = suspended.

This means that there is a side effect at might_sleep() if it is
called with CPU MSR[TS] = 0 and the task has regs->msr[TS] != 0.

This side effect can cause a TM bad thing, since at the exception
entrance, the stack saves MSR[TS]=0, and this is what will be used at
RFID, but, the processor has MSR[TS] = Suspended, and this transition
will be invalid and a TM Bad thing will be raised, causing the
following crash:

  Unexpected TM Bad Thing exception at c00000000000e9ec (msr 0x8000000302a03031) tm_scratch=800000010280b033
  cpu 0xc: Vector: 700 (Program Check) at [c00000003ff1fd70]
      pc: c00000000000e9ec: fast_exception_return+0x100/0x1bc
      lr: c000000000032948: handle_rt_signal64+0xb8/0xaf0
      sp: c0000004263ebc40
     msr: 8000000302a03031
    current = 0xc000000415050300
    paca    = 0xc00000003ffc4080	 irqmask: 0x03	 irq_happened: 0x01
      pid   = 25006, comm = sigfuz
  Linux version 5.0.0-rc1-00001-g3bd6e94bec12 (breno@debian) (gcc version 8.2.0 (Debian 8.2.0-3)) #899 SMP Mon Jan 7 11:30:07 EST 2019
  WARNING: exception is not recoverable, can't continue
  enter ? for help
  [c0000004263ebc40] c000000000032948 handle_rt_signal64+0xb8/0xaf0 (unreliable)
  [c0000004263ebd30] c000000000022780 do_notify_resume+0x2f0/0x430
  [c0000004263ebe20] c00000000000e844 ret_from_except_lite+0x70/0x74
  --- Exception: c00 (System Call) at 00007fffbaac400c
  SP (7fffeca90f40) is in userspace

The solution for this problem is running the sigreturn code with
regs->msr[TS] disabled, thus, avoiding hitting the side effect above.
This does not seem to be a problem since regs->msr will be replaced by
the ucontext value, so, it is being flushed already. In this case, it
is flushed earlier.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>