powerpc/tm: P9 disable transactionally suspended sigcontexts

Unfortunately userspace can construct a sigcontext which enables
suspend. Thus userspace can force Linux into a path where trechkpt is
executed.

This patch blocks this from happening on POWER9 by sanity checking
sigcontexts passed in.

ptrace doesn't have this problem as only MSR SE and BE can be changed
via ptrace.

This patch also adds a number of WARN_ON()s in case we ever enter
suspend when we shouldn't. This should not happen, but if it does the
symptoms are soft lockup warnings which are not obviously TM related,
so the WARN_ON()s should make it obvious what's happening.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/process.c

@@ -910,6 +910,8 @@ static inline void tm_reclaim_task(struct task_struct *tsk)
 	if (!MSR_TM_ACTIVE(thr->regs->msr))
 		goto out_and_saveregs;
 
+	WARN_ON(tm_suspend_disabled);
+
 	TM_DEBUG("--- tm_reclaim on pid %d (NIP=%lx, "
 		 "ccr=%lx, msr=%lx, trap=%lx)\n",
 		 tsk->pid, thr->regs->nip,

arch/powerpc/kernel/signal_32.c

@@ -519,6 +519,8 @@ static int save_tm_user_regs(struct pt_regs *regs,
 {
 	unsigned long msr = regs->msr;
 
+	WARN_ON(tm_suspend_disabled);
+
 	/* Remove TM bits from thread's MSR.  The MSR in the sigcontext
 	 * just indicates to userland that we were doing a transaction, but we
 	 * don't want to return in transactional state.  This also ensures
@@ -769,6 +771,8 @@ static long restore_tm_user_regs(struct pt_regs *regs,
 	int i;
 #endif
 
+	if (tm_suspend_disabled)
+		return 1;
 	/*
 	 * restore general registers but not including MSR or SOFTE. Also
 	 * take care of keeping r2 (TLS) intact if not a signal.

arch/powerpc/kernel/signal_64.c

@@ -214,6 +214,8 @@ static long setup_tm_sigcontexts(struct sigcontext __user *sc,
 
 	BUG_ON(!MSR_TM_ACTIVE(regs->msr));
 
+	WARN_ON(tm_suspend_disabled);
+
 	/* Remove TM bits from thread's MSR.  The MSR in the sigcontext
 	 * just indicates to userland that we were doing a transaction, but we
 	 * don't want to return in transactional state.  This also ensures
@@ -430,6 +432,9 @@ static long restore_tm_sigcontexts(struct task_struct *tsk,
 
 	BUG_ON(tsk != current);
 
+	if (tm_suspend_disabled)
+		return -EINVAL;
+
 	/* copy the GPRs */
 	err |= __copy_from_user(regs->gpr, tm_sc->gp_regs, sizeof(regs->gpr));
 	err |= __copy_from_user(&tsk->thread.ckpt_regs, sc->gp_regs,