FAILED: patch "[PATCH] ext4: fix race between ext4_sync_parent() and rename()" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 08adf452e628b0e2ce9a01048cfbec52353703d7 Mon Sep 17 00:00:00 2001
From: Eric Biggers <[email protected]>
Date: Wed, 6 May 2020 11:31:40 -0700
Subject: [PATCH] ext4: fix race between ext4_sync_parent() and rename()

'igrab(d_inode(dentry->d_parent))' without holding dentry->d_lock is broken because without d_lock, d_parent can be concurrently changed due to a rename(). Then if the old directory is immediately deleted, old d_parent->inode can be NULL. That causes a NULL dereference in igrab().

To fix this, use dget_parent() to safely grab a reference to the parent dentry, which pins the inode. This also eliminates the need to use d_find_any_alias() other than for the initial inode, as we no longer throw away the dentry at each step.

This is an extremely hard race to hit, but it is possible. Adding a udelay() in between the reads of ->d_parent and its ->d_inode makes it reproducible on a no-journal filesystem using the following program:

    #include <fcntl.h>
    #include <unistd.h>

    int main()
    {
        if (fork()) {
            for (;;) {
                mkdir("dir1", 0700);
                int fd = open("dir1/file", O_RDWR|O_CREAT|O_SYNC);
                write(fd, "X", 1);
                close(fd);
            }
        } else {
            mkdir("dir2", 0700);
            for (;;) {
                rename("dir1/file", "dir2/file");
                rmdir("dir1");
            }
        }
    }

index e10206e7f4bb..093c359952cd 100644
@@ -44,30 +44,28 @@
  */
 static int ext4_sync_parent(struct inode *inode)
 {
-	struct dentry *dentry = NULL;
-	struct inode *next;
+	struct dentry *dentry, *next;
 	int ret = 0;
 
 	if (!ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY))
 		return 0;
-	inode = igrab(inode);
+	dentry = d_find_any_alias(inode);
+	if (!dentry)
+		return 0;
 	while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {
 		ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);
-		dentry = d_find_any_alias(inode);
-		if (!dentry)
-			break;
-		next = igrab(d_inode(dentry->d_parent));
+
+		next = dget_parent(dentry);
 		dput(dentry);
-		if (!next)
-			break;
-		iput(inode);
-		inode = next;
+		dentry = next;
+		inode = dentry->d_inode;
+
 		/*
 		 * The directory inode may have gone through rmdir by now. But
 		 * the inode itself and its blocks are still allocated (we hold
-		 * a reference to the inode so it didn't go through
-		 * ext4_evict_inode()) and so we are safe to flush metadata
-		 * blocks and the inode.
+		 * a reference to the inode via its dentry), so it didn't go
+		 * through ext4_evict_inode()) and so we are safe to flush
+		 * metadata blocks and the inode.
 		 */
 		ret = sync_mapping_buffers(inode->i_mapping);
 		if (ret)

@@ -76,7 +74,7 @@ static int ext4_sync_parent(struct inode *inode)
 		if (ret)
 			break;
 	}
-	iput(inode);
+	dput(dentry);
 	return ret;
 }