fscrypt: don't allow v1 policies with casefolding

Casefolded encrypted directories will use a new dirhash method that
requires a secret key.  If the directory uses a v2 encryption policy,
it's easy to derive this key from the master key using HKDF.  However,
v1 encryption policies don't provide a way to derive additional keys.

Therefore, don't allow casefolding on directories that use a v1 policy.
Specifically, make it so that trying to enable casefolding on a
directory that has a v1 policy fails, trying to set a v1 policy on a
casefolded directory fails, and trying to open a casefolded directory
that has a v1 policy (if one somehow exists on-disk) fails.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
[EB: improved commit message, updated fscrypt.rst, and other cleanups]
Link: https://lore.kernel.org/r/20200120223201.241390-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/inode.c b/fs/inode.c
index 96d62d97694e..ea15c6d9f274 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -12,6 +12,7 @@
 #include <linux/security.h>
 #include <linux/cdev.h>
 #include <linux/memblock.h>
+#include <linux/fscrypt.h>
 #include <linux/fsnotify.h>
 #include <linux/mount.h>
 #include <linux/posix_acl.h>
@@ -2252,7 +2253,7 @@ int vfs_ioc_setflags_prepare(struct inode *inode, unsigned int oldflags,
 	    !capable(CAP_LINUX_IMMUTABLE))
 		return -EPERM;
 
-	return 0;
+	return fscrypt_prepare_setflags(inode, oldflags, flags);
 }
 EXPORT_SYMBOL(vfs_ioc_setflags_prepare);