fscrypt: restrict IV_INO_LBLK_* to AES-256-XTS

IV_INO_LBLK_* exist only because of hardware limitations, and currently
the only known use case for them involves AES-256-XTS.  Therefore, for
now only allow them in combination with AES-256-XTS.  This way we don't
have to worry about them being combined with other encryption modes.

(To be clear, combining IV_INO_LBLK_* with other encryption modes
*should* work just fine.  It's just not being tested, so we can't be
100% sure it works.  So with no known use case, it's best to disallow it
for now, just like we don't allow other weird combinations like
AES-256-XTS contents encryption with Adiantum filenames encryption.)

This can be relaxed later if a use case for other combinations arises.

Fixes: b103fb7653ff ("fscrypt: add support for IV_INO_LBLK_64 policies")
Fixes: e3b1078bedd3 ("fscrypt: add support for IV_INO_LBLK_32 policies")
Link: https://lore.kernel.org/r/20200721181012.39308-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 8a8ad0e44bb87923b029cb229fb394859854f8db..8e667aadf27101807e25085473003a55f6fbf6d6 100644 (file)
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -77,6 +77,20 @@ static bool supported_iv_ino_lblk_policy(const struct fscrypt_policy_v2 *policy,
        struct super_block *sb = inode->i_sb;
        int ino_bits = 64, lblk_bits = 64;
 
+       /*
+        * IV_INO_LBLK_* exist only because of hardware limitations, and
+        * currently the only known use case for them involves AES-256-XTS.
+        * That's also all we test currently.  For these reasons, for now only
+        * allow AES-256-XTS here.  This can be relaxed later if a use case for
+        * IV_INO_LBLK_* with other encryption modes arises.
+        */
+       if (policy->contents_encryption_mode != FSCRYPT_MODE_AES_256_XTS) {
+               fscrypt_warn(inode,
+                            "Can't use %s policy with contents mode other than AES-256-XTS",
+                            type);
+               return false;
+       }
+
        /*
         * It's unsafe to include inode numbers in the IVs if the filesystem can
         * potentially renumber inodes, e.g. via filesystem shrinking.