From: Eric Biggers ebiggers@google.com
commit f5e55e777cc93eae1416f0fa4908e8846b6d7825 upstream. [Please apply to 4.19-stable. This is an important fix to have, and it will be needed for xfstest generic/398 to pass if https://lkml.kernel.org/r/20201031054141.695517-1-ebiggers@kernel.org is applied. This is a clean cherry-pick to 4.19, but it doesn't apply to 4.14 and earlier; different backports would be needed for that.]
Currently, trying to rename or link a regular file, directory, or symlink into an encrypted directory fails with EPERM when the source file is unencrypted or is encrypted with a different encryption policy, and is on the same mountpoint. It is correct for the operation to fail, but the choice of EPERM breaks tools like 'mv' that know to copy rather than rename if they see EXDEV, but don't know what to do with EPERM.
Our original motivation for EPERM was to encourage users to securely handle their data. Encrypting files by "moving" them into an encrypted directory can be insecure because the unencrypted data may remain in free space on disk, where it can later be recovered by an attacker. It's much better to encrypt the data from the start, or at least try to securely delete the source data e.g. using the 'shred' program.
However, the current behavior hasn't been effective at achieving its goal because users tend to be confused, hack around it, and complain; see e.g. https://github.com/google/fscrypt/issues/76. And in some cases it's actually inconsistent or unnecessary. For example, 'mv'-ing files between differently encrypted directories doesn't work even in cases where it can be secure, such as when in userspace the same passphrase protects both directories. Yet, you *can* already 'mv' unencrypted files into an encrypted directory if the source files are on a different mountpoint, even though doing so is often insecure.
There are probably better ways to teach users to securely handle their files. For example, the 'fscrypt' userspace tool could provide a command that migrates unencrypted files into an encrypted directory, acting like 'shred' on the source files and providing appropriate warnings depending on the type of the source filesystem and disk.
Receiving errors on unimportant files might also force some users to disable encryption, thus making the behavior counterproductive. It's desirable to make encryption as unobtrusive as possible.
Therefore, change the error code from EPERM to EXDEV so that tools looking for EXDEV will fall back to a copy.
This, of course, doesn't prevent users from still doing the right things to securely manage their files. Note that this also matches the behavior when a file is renamed between two project quota hierarchies; so there's precedent for using EXDEV for things other than mountpoints.
xfstests generic/398 will require an update with this change.
[Rewritten from an earlier patch series by Michael Halcrow.]
Cc: Michael Halcrow mhalcrow@google.com Cc: Joe Richey joerichey@google.com Signed-off-by: Eric Biggers ebiggers@google.com --- Documentation/filesystems/fscrypt.rst | 12 ++++++++++-- fs/crypto/hooks.c | 6 +++--- fs/crypto/policy.c | 3 +-- include/linux/fscrypt.h | 4 ++-- 4 files changed, 16 insertions(+), 9 deletions(-)
diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst index cfbc18f0d9c98..5b667ee1242ae 100644 --- a/Documentation/filesystems/fscrypt.rst +++ b/Documentation/filesystems/fscrypt.rst @@ -426,10 +426,18 @@ astute users may notice some differences in behavior: - Unencrypted files, or files encrypted with a different encryption policy (i.e. different key, modes, or flags), cannot be renamed or linked into an encrypted directory; see `Encryption policy - enforcement`_. Attempts to do so will fail with EPERM. However, + enforcement`_. Attempts to do so will fail with EXDEV. However, encrypted files can be renamed within an encrypted directory, or into an unencrypted directory.
+ Note: "moving" an unencrypted file into an encrypted directory, e.g. + with the `mv` program, is implemented in userspace by a copy + followed by a delete. Be aware that the original unencrypted data + may remain recoverable from free space on the disk; prefer to keep + all files encrypted from the very beginning. The `shred` program + may be used to overwrite the source files but isn't guaranteed to be + effective on all filesystems and storage devices. + - Direct I/O is not supported on encrypted files. Attempts to use direct I/O on such files will fall back to buffered I/O.
@@ -516,7 +524,7 @@ not be encrypted. Except for those special files, it is forbidden to have unencrypted files, or files encrypted with a different encryption policy, in an encrypted directory tree. Attempts to link or rename such a file into -an encrypted directory will fail with EPERM. This is also enforced +an encrypted directory will fail with EXDEV. This is also enforced during ->lookup() to provide limited protection against offline attacks that try to disable or downgrade encryption in known locations where applications may later write sensitive data. It is recommended diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c index 926e5df20ec31..56debb1fcf5eb 100644 --- a/fs/crypto/hooks.c +++ b/fs/crypto/hooks.c @@ -58,7 +58,7 @@ int __fscrypt_prepare_link(struct inode *inode, struct inode *dir) return err;
if (!fscrypt_has_permitted_context(dir, inode)) - return -EPERM; + return -EXDEV;
return 0; } @@ -82,13 +82,13 @@ int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry, if (IS_ENCRYPTED(new_dir) && !fscrypt_has_permitted_context(new_dir, d_inode(old_dentry))) - return -EPERM; + return -EXDEV;
if ((flags & RENAME_EXCHANGE) && IS_ENCRYPTED(old_dir) && !fscrypt_has_permitted_context(old_dir, d_inode(new_dentry))) - return -EPERM; + return -EXDEV; } return 0; } diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c index 4288839501e94..e9d975f39f46b 100644 --- a/fs/crypto/policy.c +++ b/fs/crypto/policy.c @@ -153,8 +153,7 @@ EXPORT_SYMBOL(fscrypt_ioctl_get_policy); * malicious offline violations of this constraint, while the link and rename * checks are needed to prevent online violations of this constraint. * - * Return: 1 if permitted, 0 if forbidden. If forbidden, the caller must fail - * the filesystem operation with EPERM. + * Return: 1 if permitted, 0 if forbidden. */ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child) { diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h index 952ab97af325e..9fa48180a1f5f 100644 --- a/include/linux/fscrypt.h +++ b/include/linux/fscrypt.h @@ -89,7 +89,7 @@ static inline int fscrypt_require_key(struct inode *inode) * in an encrypted directory tree use the same encryption policy. * * Return: 0 on success, -ENOKEY if the directory's encryption key is missing, - * -EPERM if the link would result in an inconsistent encryption policy, or + * -EXDEV if the link would result in an inconsistent encryption policy, or * another -errno code. */ static inline int fscrypt_prepare_link(struct dentry *old_dentry, @@ -119,7 +119,7 @@ static inline int fscrypt_prepare_link(struct dentry *old_dentry, * We also verify that the rename will not violate the constraint that all files * in an encrypted directory tree use the same encryption policy. * - * Return: 0 on success, -ENOKEY if an encryption key is missing, -EPERM if the + * Return: 0 on success, -ENOKEY if an encryption key is missing, -EXDEV if the * rename would cause inconsistent encryption policies, or another -errno code. */ static inline int fscrypt_prepare_rename(struct inode *old_dir,