mm: fix uninitialized use in overcommit_policy_handler

BugLink: https://bugs.launchpad.net/bugs/1949640
commit bcbda81020c3ee77e2c098cadf3e84f99ca3de17 upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/mm/util.c b/mm/util.c
index 8c9b7d1e7c499828c429b93853af28e7299edcc1..ff51b7d23d3b05279bae1d1f9eac6c487ec82722 100644 (file)
--- a/mm/util.c
+++ b/mm/util.c
@@ -768,7 +768,7 @@ 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;
 
        /*
@@ -786,7 +786,7 @@ 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);