Re: [PATCH v2 1/1] mm: Fix the uninitialized use in overcommit_policy_handler

[Michal Hocko]

On Wed 22-09-21 01:41:22, Chen Jun wrote:
> An unexpected value of /proc/sys/vm/panic_on_oom we will get,
> after running the following program.
> 
> int main()
> {
>     int fd = open("/proc/sys/vm/panic_on_oom", 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.
> 
> Fixes: 56f3547bfa4d ("mm: adjust vm_committed_as_batch according to vm overcommit policy"
> Signed-off-by: Chen Jun <chenjun102@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Btw. you could also check for new_policy == sysctl_overcommit_memory so
that the sync of pcp counters is not fired without the policy change.
Something for a separate patch I guess.

> ---
> 
> v2:
>   * Check whether new_policy has been changed by proc_dointvec_minmax.
> 
>  mm/util.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 4ddb6e186dd5..d5be67771850 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -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

-- 
Michal Hocko
SUSE Labs