membarrier: Explicitly sync remote cores when SYNC_CORE is requested

commit 758c937 upstream.

membarrier() does not explicitly sync_core() remote CPUs; instead, it
relies on the assumption that an IPI will result in a core sync.  On x86,
this may be true in practice, but it's not architecturally reliable.  In
particular, the SDM and APM do not appear to guarantee that interrupt
delivery is serializing.  While IRET does serialize, IPI return can
schedule, thereby switching to another task in the same mm that was
sleeping in a syscall.  The new task could then SYSRET back to usermode
without ever executing IRET.

Make this more robust by explicitly calling sync_core_before_usermode()
on remote cores.  (This also helps people who search the kernel tree for
instances of sync_core() and sync_core_before_usermode() -- one might be
surprised that the core membarrier code doesn't currently show up in a
such a search.)

Fixes: 70216e1 ("membarrier: Provide core serializing command, *_SYNC_CORE")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/776b448d5f7bd6b12690707f5ed67bcda7f1d427.1607058304.git.luto@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/sched/membarrier.c

@@ -30,6 +30,23 @@ static void ipi_mb(void *info)
 	smp_mb();	/* IPIs should be serializing but paranoid. */
 }
 
+static void ipi_sync_core(void *info)
+{
+	/*
+	 * The smp_mb() in membarrier after all the IPIs is supposed to
+	 * ensure that memory on remote CPUs that occur before the IPI
+	 * become visible to membarrier()'s caller -- see scenario B in
+	 * the big comment at the top of this file.
+	 *
+	 * A sync_core() would provide this guarantee, but
+	 * sync_core_before_usermode() might end up being deferred until
+	 * after membarrier()'s smp_mb().
+	 */
+	smp_mb();	/* IPIs should be serializing but paranoid. */
+
+	sync_core_before_usermode();
+}
+
 static void ipi_sync_rq_state(void *info)
 {
 	struct mm_struct *mm = (struct mm_struct *) info;
@@ -134,13 +151,15 @@ static int membarrier_private_expedited(int flags)
 	int cpu;
 	cpumask_var_t tmpmask;
 	struct mm_struct *mm = current->mm;
+	smp_call_func_t ipi_func = ipi_mb;
 
 	if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
 		if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
 			return -EINVAL;
 		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
 			return -EPERM;
+		ipi_func = ipi_sync_core;
 	} else {
 		if (!(atomic_read(&mm->membarrier_state) &
 		      MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
@@ -181,7 +200,7 @@ static int membarrier_private_expedited(int flags)
 	rcu_read_unlock();
 
 	preempt_disable();
-	smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+	smp_call_function_many(tmpmask, ipi_func, NULL, 1);
 	preempt_enable();
 
 	free_cpumask_var(tmpmask);