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

[NeilBrown]

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@xxxxxxxx>
---
 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;
 		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)) {