Re: [PATCH 3/3] VFS: close race between getcwd() and d_move()

[Nikolay Borisov <nborisov@suse.com>]

On  9.11.2017 05:22, NeilBrown wrote:
> d_move() will call __d_drop() and then __d_rehash()
> on the dentry being moved.  This creates a small window
> when the dentry appears to be unhashed.  Many tests
> of d_unhashed() are made under ->d_lock and so are safe
> from racing with this window, but some aren't.
> In particular, getcwd() calls d_unlinked() (which calls
> d_unhashed()) without d_lock protection, so it can race.
> 
> This races has been seen in practice with lustre, which uses d_move() as
> part of name lookup.  See:
>    https://jira.hpdd.intel.com/browse/LU-9735
> It could race with a regular rename(), and result in ENOENT instead
> of either the 'before' or 'after' name.
> 
> The race can be demonstrated with a simple program which
> has two threads, one renaming a directory back and forth
> while another calls getcwd() within that directory: it should never
> fail, but does.  See:
>   https://patchwork.kernel.org/patch/9455345/
> 
> We could fix this race by taking d_lock and rechecking when
> d_unhashed() reports true.  Alternately when can remove the window,
> which is the approach this patch takes.
> 
> When __d_drop and __d_rehash are used to move a dentry, an extra
> flag is passed which causes d_hash.pprev to not be cleared, and
> to not be tested.
> 
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>  fs/dcache.c |   31 ++++++++++++++++++++-----------
>  1 file changed, 20 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index d5952306206b..3130d62f29c9 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -471,8 +471,11 @@ static void dentry_lru_add(struct dentry *dentry)
>   * reason (NFS timeouts or autofs deletes).
>   *
>   * __d_drop requires dentry->d_lock.
> + * ___d_drop takes an extra @moving argument.
> + * If true, d_hash.pprev is not cleared, so there is no transient d_unhashed()
> + * state.
>   */
> -void __d_drop(struct dentry *dentry)
> +static void inline ___d_drop(struct dentry *dentry, bool moving)
>  {
>  	if (!d_unhashed(dentry)) {
>  		struct hlist_bl_head *b;
> @@ -493,12 +496,18 @@ void __d_drop(struct dentry *dentry)
>  		} else
>  			hlist_bl_lock(b);
>  		__hlist_bl_del(&dentry->d_hash);
> -		dentry->d_hash.pprev = NULL;
> +		if (likely(!moving))
> +			dentry->d_hash.pprev = NULL;

nit: isn't a bit more explicit if (unlikely(moving)). I suspect the end
result is the same, however it's easy to miss the !. It's not a big deal
but just wondering.

>  		hlist_bl_unlock(b);
>  		/* After this call, in-progress rcu-walk path lookup will fail. */
>  		write_seqcount_invalidate(&dentry->d_seq);
>  	}
>  }
> +
> +void __d_drop(struct dentry *dentry)
> +{
> +	___d_drop(dentry, false);
> +}
>  EXPORT_SYMBOL(__d_drop);
>  
>  void d_drop(struct dentry *dentry)
> @@ -2387,10 +2396,10 @@ void d_delete(struct dentry * dentry)
>  }
>  EXPORT_SYMBOL(d_delete);
>  
> -static void __d_rehash(struct dentry *entry)
> +static void __d_rehash(struct dentry *entry, bool moving)
>  {
>  	struct hlist_bl_head *b = d_hash(entry->d_name.hash);
> -	BUG_ON(!d_unhashed(entry));
> +	BUG_ON(!moving && !d_unhashed(entry));
>  	hlist_bl_lock(b);
>  	hlist_bl_add_head_rcu(&entry->d_hash, b);
>  	hlist_bl_unlock(b);
> @@ -2406,7 +2415,7 @@ static void __d_rehash(struct dentry *entry)
>  void d_rehash(struct dentry * entry)
>  {
>  	spin_lock(&entry->d_lock);
> -	__d_rehash(entry);
> +	__d_rehash(entry, false);
>  	spin_unlock(&entry->d_lock);
>  }
>  EXPORT_SYMBOL(d_rehash);
> @@ -2580,7 +2589,7 @@ static inline void __d_add(struct dentry *dentry, struct inode *inode)
>  		raw_write_seqcount_end(&dentry->d_seq);
>  		fsnotify_update_flags(dentry);
>  	}
> -	__d_rehash(dentry);
> +	__d_rehash(dentry, false);
>  	if (dir)
>  		end_dir_add(dir, n);
>  	spin_unlock(&dentry->d_lock);
> @@ -2642,7 +2651,7 @@ struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
>  			alias = NULL;
>  		} else {
>  			__dget_dlock(alias);
> -			__d_rehash(alias);
> +			__d_rehash(alias, false);
>  			spin_unlock(&alias->d_lock);
>  		}
>  		spin_unlock(&inode->i_lock);
> @@ -2828,8 +2837,8 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
>  
>  	/* unhash both */
>  	/* __d_drop does write_seqcount_barrier, but they're OK to nest. */
> -	__d_drop(dentry);
> -	__d_drop(target);
> +	___d_drop(dentry, true);
> +	___d_drop(target, exchange);
>  
>  	/* Switch the names.. */
>  	if (exchange)
> @@ -2838,9 +2847,9 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
>  		copy_name(dentry, target);
>  
>  	/* rehash in new place(s) */
> -	__d_rehash(dentry);
> +	__d_rehash(dentry, true);
>  	if (exchange)
> -		__d_rehash(target);
> +		__d_rehash(target, true);
>  
>  	/* ... and switch them in the tree */
>  	if (IS_ROOT(dentry)) {
> 
> 
>