ext4: fix races in ext4_sync_parent()

Fix problems if fsync() races against a rename of a parent directory
as pointed out by Al Viro in his own inimitable way:

>While we are at it, could somebody please explain what the hell is ext4
>doing in
>static int ext4_sync_parent(struct inode *inode)
>{
>        struct writeback_control wbc;
>        struct dentry *dentry = NULL;
>        int ret = 0;
>
>        while (inode && ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
>                ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
>                dentry = list_entry(inode->i_dentry.next,
>                                    struct dentry, d_alias);
>                if (!dentry || !dentry->d_parent || !dentry->d_parent->d_inode)
>                        break;
>                inode = dentry->d_parent->d_inode;
>                ret = sync_mapping_buffers(inode->i_mapping);
>                ...
>Note that dentry obviously can't be NULL there.  dentry->d_parent is never
>NULL.  And dentry->d_parent would better not be negative, for crying out
>loud!  What's worse, there's no guarantees that dentry->d_parent will
>remain our parent over that sync_mapping_buffers() *and* that inode won't
>just be freed under us (after rename() and memory pressure leading to
>eviction of what used to be our dentry->d_parent)......

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 21 insertions, 5 deletions

diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index ce66d2fe826c..f9dbe33cde5e 100644
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -129,15 +129,30 @@ static int ext4_sync_parent(struct inode *inode)
 {
         struct writeback_control wbc;
         struct dentry *dentry = NULL;
+        struct inode *next;
         int ret = 0;
 
-        while (inode && ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
+        if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
+                return 0;
+        inode = igrab(inode);
+        while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
                 ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
-                dentry = list_entry(inode->i_dentry.next,
-                                    struct dentry, d_alias);
-                if (!dentry || !dentry->d_parent || !dentry->d_parent->d_inode)
+                dentry = NULL;
+                spin_lock(&inode->i_lock);
+                if (!list_empty(&inode->i_dentry)) {
+                        dentry = list_first_entry(&inode->i_dentry,
+                                                  struct dentry, d_alias);
+                        dget(dentry);
+                }
+                spin_unlock(&inode->i_lock);
+                if (!dentry)
+                        break;
+                next = igrab(dentry->d_parent->d_inode);
+                dput(dentry);
+                if (!next)
                         break;
-                inode = dentry->d_parent->d_inode;
+                iput(inode);
+                inode = next;
                 ret = sync_mapping_buffers(inode->i_mapping);
                 if (ret)
                         break;
@@ -148,6 +163,7 @@ static int ext4_sync_parent(struct inode *inode)
                 if (ret)
                         break;
         }
+        iput(inode);
         return ret;
 }