Commit 08adf452 authored by Eric Biggers's avatar Eric Biggers Committed by Theodore Ts'o
Browse files

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");
            }
        }
    }

Fixes: d59729f4 ("ext4: fix races in ext4_sync_parent()")
Cc: stable@vger.kernel.org
Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200506183140.541194-1-ebiggers@kernel.org


Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 80dd4978

fs/ext4/fsync.c

+13 −15
Original line number Diff line number Diff line
@@ -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);

	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));

		return 0;

	while (ext4_test_inode_state(inode, EXT4_STATE_NEWENTRY)) {

		ext4_clear_inode_state(inode, EXT4_STATE_NEWENTRY);



		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;

}