linux-ext4 - Re: [PATCH v2] ext4: forbid encrypting root directory

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Message-Id: <CFA0E924-C86A-4B01-8104-4B89FDCDF10E@dilger.ca>
Date:   Mon, 19 Jun 2017 15:18:24 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v2] ext4: forbid encrypting root directory

On Jun 16, 2017, at 12:34 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> 
> From: Eric Biggers <ebiggers@...gle.com>
> 
> Currently it's possible to encrypt all files and directories on an ext4
> filesystem by deleting everything, including lost+found, then setting an
> encryption policy on the root directory.  However, this is incompatible
> with e2fsck because e2fsck expects to find, create, and/or write to
> lost+found and does not have access to any encryption keys.  Especially
> problematic is that if e2fsck can't find lost+found, it will create it
> without regard for whether the root directory is encrypted.  This is
> wrong for obvious reasons, and it causes a later run of e2fsck to
> consider the lost+found directory entry to be corrupted.
> 
> Encrypting the root directory may also be of limited use because it is
> the "all-or-nothing" use case, for which dm-crypt can be used instead.
> (By design, encryption policies are inherited and cannot be overridden;
> so the root directory having an encryption policy implies that all files
> and directories on the filesystem have that same encryption policy.)
> 
> In any case, encrypting the root directory is broken currently and must
> not be allowed; so start returning an error if userspace requests it.
> For now only do this in ext4, because f2fs and ubifs do not appear to
> have the lost+found requirement.  We could move it into
> fscrypt_ioctl_set_policy() later if desired, though.
> 
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Reviewed-by: Andreas Dilger <adilger@...ger.ca>

> ---
> 
> v2: use EPERM instead of EBUSY, and tweak commit message
> 
> fs/ext4/super.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index d37c81f327e7..d5b5c80c23f5 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1145,6 +1145,15 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
> 	handle_t *handle = fs_data;
> 	int res, res2, retries = 0;
> 
> +	/*
> +	 * Encrypting the root directory is not allowed because e2fsck expects
> +	 * lost+found to exist and be unencrypted, and encrypting the root
> +	 * directory would imply encrypting the lost+found directory as well as
> +	 * the filename "lost+found" itself.
> +	 */
> +	if (inode->i_ino == EXT4_ROOT_INO)
> +		return -EPERM;
> +
> 	res = ext4_convert_inline_data(inode);
> 	if (res)
> 		return res;
> --
> 2.13.1.518.g3df882009-goog
> 


Cheers, Andreas






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