fscrypto: Return -EXDEV for link, rename, and cross-rename between incompat contexts

Currently file systems support fscrypto will return -EPERM when the user attempts to link, rename, or cross-rename between two directories that have incompatible encryption policy contexts. User space tools will fail the operation when receiving this errno. With -EXDEV, user space tools will typically fall back to copy-and-delete instead.

Our original motivation for returning -EPERM was to force users to try harder when doing these operations, hopefully making them think more carefully about whether what they're doing is secure. One security concern is that when moving files between unencrypted locations into encrypted locations, the data in the unencrypted location will remain in the clear on the storage device until the freed blocks are overwritten at some arbitrary point in the future (if ever). Moving files from encrypted locations into unencrypted locations is also (perhaps more obviously) problematic.

Whether making things fail will have the intended effect on users is up for debate. Meanwhile I've had at least one person tell me their userspace tools are failing and that they would prefer seeing the same sort of behavior that they see when (for example) moving files from one project quota hierarchy to another (ext4 returns -EXDEV).

Note that xfstests generic/398 will require an update with this change.

Michael Halcrow (3):
ext4 crypto: Return -EXDEV for link, rename, and cross-rename between
incompat contexts
F2FS crypto: Return -EXDEV for link, rename, and cross-rename between
incompat contexts
UBIFS crypto: Return -EXDEV for link, rename, and cross-rename between
incompat contexts

 fs/ext4/namei.c | 6 +++---
 fs/f2fs/namei.c | 6 +++---
 fs/ubifs/dir.c  | 6 +++---
 3 files changed, 9 insertions(+), 9 deletions(-)

2.14.1.581.gf28d330327-goog