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

Eric Biggers
2017-10-08T19:17:52Z

Hi Ted,

On Sun, Oct 08, 2017 at 02:09:53PM -0400, Theodore Ts'o wrote:
> diff --git a/fs/ext4/crypto_key.c b/fs/ext4/crypto_key.c
> index 52170d0b7c40..c8c3bf0286be 100644
> --- a/fs/ext4/crypto_key.c
> +++ b/fs/ext4/crypto_key.c
> @@ -99,9 +99,14 @@ 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;
> +
> + if (ext4_has_encryption_key(inode))
> + 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)

This is an improvement, but it's still broken because there is no locking. If
two processes ->open() the same inode at the same time, one can observe
ext4_has_encryption_key() as true before the key has been fully initialized. It
may be a good idea to add a really simple
DEFINE_MUTEX(ext4_key_generation_mutex), then do the ext4_has_encryption_key()
check and key generation under the mutex...

Eric