hugetlb: check the return value of string conversion in sysctl handler

proc_doulongvec_minmax may fail if the given buffer doesn't represent a
valid number.  If we provide something invalid we will initialize the
resulting value (nr_overcommit_huge_pages in this case) to a random value
from the stack.

The issue was introduced by a3d0c6a when the default handler has been
replaced by the helper function where we do not check the return value.

Reproducer:
echo "" > /proc/sys/vm/nr_overcommit_hugepages

[akpm@linux-foundation.org: correctly propagate proc_doulongvec_minmax return code]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: CAI Qian <caiqian@redhat.com>
Cc: Nishanth Aravamudan <nacc@us.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -1859,13 +1859,16 @@ static int hugetlb_sysctl_handler_common(bool obey_mempolicy,
 {
 	struct hstate *h = &default_hstate;
 	unsigned long tmp;
+	int ret;
 
 	if (!write)
 		tmp = h->max_huge_pages;
 
 	table->data = &tmp;
 	table->maxlen = sizeof(unsigned long);
-	proc_doulongvec_minmax(table, write, buffer, length, ppos);
+	ret = proc_doulongvec_minmax(table, write, buffer, length, ppos);
+	if (ret)
+		goto out;
 
 	if (write) {
 		NODEMASK_ALLOC(nodemask_t, nodes_allowed,
@@ -1880,8 +1883,8 @@ static int hugetlb_sysctl_handler_common(bool obey_mempolicy,
 		if (nodes_allowed != &node_states[N_HIGH_MEMORY])
 			NODEMASK_FREE(nodes_allowed);
 	}
-
-	return 0;
+out:
+	return ret;
 }
 
 int hugetlb_sysctl_handler(struct ctl_table *table, int write,
@@ -1919,21 +1922,24 @@ int hugetlb_overcommit_handler(struct ctl_table *table, int write,
 {
 	struct hstate *h = &default_hstate;
 	unsigned long tmp;
+	int ret;
 
 	if (!write)
 		tmp = h->nr_overcommit_huge_pages;
 
 	table->data = &tmp;
 	table->maxlen = sizeof(unsigned long);
-	proc_doulongvec_minmax(table, write, buffer, length, ppos);
+	ret = proc_doulongvec_minmax(table, write, buffer, length, ppos);
+	if (ret)
+		goto out;
 
 	if (write) {
 		spin_lock(&hugetlb_lock);
 		h->nr_overcommit_huge_pages = tmp;
 		spin_unlock(&hugetlb_lock);
 	}
-
-	return 0;
+out:
+	return ret;
 }
 
 #endif /* CONFIG_SYSCTL */