MIPS: fork: Fix MSA/FPU/DSP context duplication race

MIPS: fork: Fix MSA/FPU/DSP context duplication race

There is a race in the MIPS fork code which allows the child to get a
stale copy of parent MSA/FPU/DSP state that is active in hardware
registers when the fork() is called. This is because copy_thread() saves
the live register state into the child context only if the hardware is
currently in use, apparently on the assumption that the hardware state
cannot have been saved and disabled since the initial duplication of the
task_struct. However preemption is certainly possible during this
window.

An example sequence of events is as follows:

1) The parent userland process puts important data into saved floating
   point registers ($f20-$f31), which are then dirty compared to the
   process' stored context.

2) The parent process calls fork() which does a clone system call.

3) In the kernel, do_fork() -> copy_process() -> dup_task_struct() ->
   arch_dup_task_struct() (which uses the weakly defined default
   implementation). This duplicates the parent process' task context,
   which includes a stale version of its FP context from when it was
   last saved, probably some time before (1).

4) At some point before copy_process() calls copy_thread(), such as when
   duplicating the memory map, the process is desceduled. Perhaps it is
   preempted asynchronously, or perhaps it sleeps while blocked on a
   mutex. The dirty FP state in the FP registers is saved to the parent
   process' context and the FPU is disabled.

5) When the process is rescheduled again it continues copying state
   until it gets to copy_thread(), which checks whether the FPU is in
   use, so that it can copy that dirty state to the child process' task
   context. Because of the deschedule however the FPU is not in use, so
   the child process' context is left with stale FP context from the
   last time the parent saved it (some time before (1)).

6) When the new child process is scheduled it reads the important data
   from the saved floating point register, and ends up doing a NULL
   pointer dereference as a result of the stale data.

This use of saved floating point registers across function calls can be
triggered fairly easily by explicitly using inline asm with a current
(MIPS R2) compiler, but is far more likely to happen unintentionally
with a MIPS R6 compiler where the FP registers are more likely to get
used as scratch registers for storing non-fp data.

It is easily fixed, in the same way that other architectures do it, by
overriding the implementation of arch_dup_task_struct() to sync the
dirty hardware state to the parent process' task context *prior* to
duplicating it, rather than copying straight to the child process' task
context in copy_thread(). Note, the FPU hardware is not disabled so the
parent process may continue executing with the live register context,
but now the child process is guaranteed to have an identical copy of it
at that point.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reported-by: Matthew Fortune <matthew.fortune@imgtec.com>
Tested-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9075/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>