• Eric Biggers's avatar
    fscrypt: fix race where ->lookup() marks plaintext dentry as ciphertext · b01531db
    Eric Biggers authored
    ->lookup() in an encrypted directory begins as follows:
    
    1. fscrypt_prepare_lookup():
        a. Try to load the directory's encryption key.
        b. If the key is unavailable, mark the dentry as a ciphertext name
           via d_flags.
    2. fscrypt_setup_filename():
        a. Try to load the directory's encryption key.
        b. If the key is available, encrypt the name (treated as a plaintext
           name) to get the on-disk name.  Otherwise decode the name
           (treated as a ciphertext name) to get the on-disk name.
    
    But if the key is concurrently added, it may be found at (2a) but not at
    (1a).  In this case, the dentry will be wrongly marked as a ciphertext
    name even though it was actually treated as plaintext.
    
    This will cause the dentry to be wrongly invalidated on the next lookup,
    potentially causing problems.  For example, if the racy ->lookup() was
    part of sys_mount(), then the new mount will be detached when anything
    tries to access it.  This is despite the mountpoint having a plaintext
    path, which should remain valid now that the key was added.
    
    Of course, this is only possible if there's a userspace race.  Still,
    the additional kernel-side race is confusing and unexpected.
    
    Close the kernel-side race by changing fscrypt_prepare_lookup() to also
    set the on-disk filename (step 2b), consistent with the d_flags update.
    
    Fixes: 28b4c263 ("ext4 crypto: revalidate dentry after adding or removing the key")
    Signed-off-by: default avatarEric Biggers <ebiggers@google.com>
    Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
    b01531db
dir.c 42.9 KB