ext4: fix race between ext4_sync_parent() and rename()

commit 08adf452e628b0e2ce9a01048cfbec52353703d7 upstream.

'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");
            }
        }
    }

Fixes: d59729f4e794 ("ext4: fix races in ext4_sync_parent()")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200506183140.541194-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/ext4/fsync.c b/fs/ext4/fsync.c
index e10206e7f4bbe7f470fd83687ddd3c48322bc892..093c359952cdbad6413d361e021555e28a318523 100644 (file)
--- a/fs/ext4/fsync.c
+++ b/fs/ext4/fsync.c
@@ -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;
 }