mm: Fix the uninitialized use in overcommit_policy_handler

An unexpected value of /proc/sys/vm/overcommit_memory we will get, after running the following program.

int main()

{
    int fd = open("/proc/sys/vm/overcommit_memory", O_RDWR)
    write(fd, "1", 1);
    write(fd, "2", 1);
    close(fd);
}

write(fd, "2", 1) will pass *ppos = 1 to proc_dointvec_minmax. proc_dointvec_minmax will return 0 without setting new_policy.

t.data = &new_policy;
ret = proc_dointvec_minmax(&t, write, buffer, lenp, ppos)

      -->do_proc_dointvec
         -->__do_proc_dointvec
              if (write) {
                if (proc_first_pos_non_zero_ignore(ppos, table))
                  goto out;

sysctl_overcommit_memory = new_policy;

so sysctl_overcommit_memory will be set to an uninitialized value.

Check whether new_policy has been changed by proc_dointvec_minmax.

---

v3:
* Correct commit message.

v2:
* Check whether new_policy has been changed by proc_dointvec_minmax.

 mm/util.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 4ddb6e186dd5..d5be67771850 100644
@@ -756,7 +756,7 @@ int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer,
 		size_t *lenp, loff_t *ppos)
 {
 	struct ctl_table t;
-	int new_policy;
+	int new_policy = -1;
 	int ret;
 
 	/*

@@ -774,7 +774,7 @@ int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer,
 		t = *table;
 		t.data = &new_policy;
 		ret = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
-		if (ret)
+		if (ret || new_policy == -1)
 			return ret;
 
 		mm_compute_batch(new_policy);

2.17.1

Thanks. I added a cc:stable to this, so the fix will be backported into earlier kernels.

Hi Andrew，

missing  ';' here,

...  and the Fixes tag miss a ')' in the end ,  please help to add them :)