[patch 16/16] mm: fix uninitialized use in overcommit_policy_handler

[Andrew Morton <akpm@linux-foundation.org>]

From: Chen Jun <chenjun102@huawei.com>
Subject: mm: fix uninitialized use in overcommit_policy_handler

We get an unexpected value of /proc/sys/vm/overcommit_memory 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.

Link: https://lkml.kernel.org/r/20210923020524.13289-1-chenjun102@huawei.com
Fixes: 56f3547bfa4d ("mm: adjust vm_committed_as_batch according to vm overcommit policy")
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Feng Tang <feng.tang@intel.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Rui Xiang <rui.xiang@huawei.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/util.c~mm-fix-the-uninitialized-use-in-overcommit_policy_handler
+++ a/mm/util.c
@@ -787,7 +787,7 @@ int overcommit_policy_handler(struct ctl
 		size_t *lenp, loff_t *ppos)
 {
 	struct ctl_table t;
-	int new_policy;
+	int new_policy = -1;
 	int ret;
 
 	/*
@@ -805,7 +805,7 @@ int overcommit_policy_handler(struct ctl
 		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);
_