Re: [PATCH v2] ovl: fix dentry leak in ovl_get_redirect

[Joseph Qi]

Hi Viro,

On 12/21/20 2:26 PM, Al Viro wrote:
> On Sun, Dec 20, 2020 at 08:09:27PM +0800, Liangyan wrote:
> 
>> +++ b/fs/overlayfs/dir.c
>> @@ -973,6 +973,7 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>>  	for (d = dget(dentry); !IS_ROOT(d);) {
>>  		const char *name;
>>  		int thislen;
>> +		struct dentry *parent = NULL;
>>  
>>  		spin_lock(&d->d_lock);
>>  		name = ovl_dentry_get_redirect(d);
>> @@ -992,7 +993,22 @@ static char *ovl_get_redirect(struct dentry *dentry, bool abs_redirect)
>>  
>>  		buflen -= thislen;
>>  		memcpy(&buf[buflen], name, thislen);
>> -		tmp = dget_dlock(d->d_parent);
>> +		parent = d->d_parent;
>> +		if (unlikely(!spin_trylock(&parent->d_lock))) {
>> +			rcu_read_lock();
>> +			spin_unlock(&d->d_lock);
>> +again:
>> +			parent = READ_ONCE(d->d_parent);
>> +			spin_lock(&parent->d_lock);
>> +			if (unlikely(parent != d->d_parent)) {
>> +				spin_unlock(&parent->d_lock);
>> +				goto again;
>> +			}
>> +			rcu_read_unlock();
>> +			spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
>> +		}
>> +		tmp = dget_dlock(parent);
>> +		spin_unlock(&parent->d_lock);
>>  		spin_unlock(&d->d_lock);
> 
> Yecchhhh....  What's wrong with just doing
> 		spin_unlock(&d->d_lock);
> 		parent = dget_parent(d);
> 		dput(d);
> 		d = parent;
> instead of that?
> 

Now race happens on non-RCU path in lookup_fast(), I'm afraid d_seq can
not close the race window.

Thanks,
Joseph