powerpc: Never handle VSX alignment exceptions from kernel

The VSX alignment handler needs to write out the existing VSX
state to memory before operating on it (flush_vsx_to_thread()).
If we take a VSX alignment exception in the kernel bad things
will happen. It looks like we could write the kernel state out
to the user process, or we could handle the kernel exception
using data from the user process (depending if MSR_VSX is set
or not).

Worse still, if the code to read or write the VSX state causes an
alignment exception, we will recurse forever. I ended up with
hundreds of megabytes of kernel stack to look through as a result.

Floating point and SPE code have similar issues but already include
a user check. Add the same check to emulate_vsx().

With this patch any unaligned VSX loads and stores in the kernel
will show up as a clear oops rather than silent corruption of
kernel or userspace VSX state, or worse, corruption of a potentially
unlimited amount of kernel memory.
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/align.c

@@ -651,6 +651,10 @@ static int emulate_vsx(unsigned char __user *addr, unsigned int reg,
 	int sw = 0;
 	int i, j;
 
+	/* userland only */
+	if (unlikely(!user_mode(regs)))
+		return 0;
+
 	flush_vsx_to_thread(current);
 
 	if (reg < 32)