Re: [FOR STABLE 4.1 -v2] ext4 crypto: don't regenerate the per-inode encryption key unnecessarily

Eric Biggers
2017-10-08T22:16:08Z

On Sun, Oct 08, 2017 at 05:55:30PM -0400, Theodore Ts'o wrote:
> diff --git a/fs/ext4/crypto_key.c b/fs/ext4/crypto_key.c
> index 52170d0b7c40..4f9818719d61 100644
> --- a/fs/ext4/crypto_key.c
> +++ b/fs/ext4/crypto_key.c
> @@ -99,9 +99,17 @@ int ext4_generate_encryption_key(struct inode *inode)
> struct ext4_encryption_context ctx;
> struct user_key_payload *ukp;
> struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> - int res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
> - EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
> - &ctx, sizeof(ctx));
> + int res;
> +
> + mutex_lock(&ei->i_encryption_lock);
> + if (ext4_has_encryption_key(inode)) {
> + mutex_unlock(&ei->i_encryption_lock);
> + return 0;
> + }
> +
> + res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
> + EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
> + &ctx, sizeof(ctx));
>
> if (res != sizeof(ctx)) {
> if (res > 0)
> @@ -154,6 +162,7 @@ out:
> key_put(keyring_key);
> if (res < 0)
> crypt_key->mode = EXT4_ENCRYPTION_MODE_INVALID;
> + mutex_unlock(&ei->i_encryption_lock);
> return res;

This looks better, but maybe the call to ext4_has_encryption_key() should also
be removed from ext4_get_fname_crypto_ctx() --- so it calls
ext4_generate_encryption_key() unconditionally, like ext4_file_open() and
ext4_file_mmap() do? Otherwise it will still be doing a racy check without the
lock.

Eric