powerpc/signal64: Don't read sigaction arguments back from user memory

When delivering a signal to a sigaction style handler (SA_SIGINFO), we
pass pointers to the siginfo and ucontext via r4 and r5.

Currently we populate the values in those registers by reading the
pointers out of the sigframe in user memory, even though the values in
user memory were written by the kernel just prior:

  unsafe_put_user(&frame->info, &frame->pinfo, badframe_block);
  unsafe_put_user(&frame->uc, &frame->puc, badframe_block);
  ...
  if (ksig->ka.sa.sa_flags & SA_SIGINFO) {
  	err |= get_user(regs->gpr[4], (unsigned long __user *)&frame->pinfo);
  	err |= get_user(regs->gpr[5], (unsigned long __user *)&frame->puc);

ie. we write &frame->info into frame->pinfo, and then read frame->pinfo
back into r4, and similarly for &frame->uc.

The code has always been like this, since linux-fullhistory commit
d4f2d95eca2c ("Forward port of 2.4 ppc64 signal changes.").

There's no reason for us to read the values back from user memory,
rather than just setting the value in the gpr[4/5] directly. In fact
reading the value back from user memory opens up the possibility of
another user thread changing the values before we read them back.
Although any process doing that would be racing against the kernel
delivering the signal, and would risk corrupting the stack, so that
would be a userspace bug.

Note that this is 64-bit only code, so there's no subtlety with the size
of pointers differing between kernel and user. Also the frame variable
is not modified to point elsewhere during the function.

In the past reading the values back from user memory was not costly, but
now that we have KUAP on some CPUs it is, so we'd rather avoid it for
that reason too.

So change the code to just set the values directly, using the same
values we have written to the sigframe previously in the function.

Note also that this matches what our 32-bit signal code does.

Using a version of will-it-scale's signal1_threads that sets SA_SIGINFO,
this results in a ~4% increase in signals per second on a Power9, from
229,777 to 239,766.
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210610072949.3198522-1-mpe@ellerman.id.au

arch/powerpc/kernel/signal_64.c

@@ -944,8 +944,8 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 	regs->gpr[3] = ksig->sig;
 	regs->result = 0;
 	if (ksig->ka.sa.sa_flags & SA_SIGINFO) {
-		err |= get_user(regs->gpr[4], (unsigned long __user *)&frame->pinfo);
-		err |= get_user(regs->gpr[5], (unsigned long __user *)&frame->puc);
+		regs->gpr[4] = (unsigned long)&frame->info;
+		regs->gpr[5] = (unsigned long)&frame->uc;
 		regs->gpr[6] = (unsigned long) frame;
 	} else {
 		regs->gpr[4] = (unsigned long)&frame->uc.uc_mcontext;