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    Subject[PATCH 4.14 147/190] ext4: fix race between ext4_sync_parent() and rename()
    Date
    From: Eric Biggers <ebiggers@google.com>

    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>

    ---
    fs/ext4/fsync.c | 28 +++++++++++++---------------
    1 file changed, 13 insertions(+), 15 deletions(-)

    --- 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
    if (ret)
    break;
    }
    - iput(inode);
    + dput(dentry);
    return ret;
    }


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