fscrypt: restrict IV_INO_LBLK_32 to ino_bits <= 32

From: Eric Biggers <[email protected]>

When an encryption policy has the IV_INO_LBLK_32 flag set, the IV generation method involves hashing the inode number. This is different from fscrypt's other IV generation methods, where the inode number is either not used at all or is included directly in the IVs.

Therefore, in principle IV_INO_LBLK_32 can work with any length inode number. However, currently fscrypt gets the inode number from inode::i_ino, which is 'unsigned long'. So currently the implementation limit is actually 32 bits (like IV_INO_LBLK_64), since longer inode numbers will have been truncated by the VFS on 32-bit platforms.

Fix fscrypt_supported_v2_policy() to enforce the correct limit.

This doesn't actually matter currently, since only ext4 and f2fs support IV_INO_LBLK_32, and they both only support 32-bit inode numbers. But we might as well fix it in case it matters in the future.

Ideally inode::i_ino would instead be made 64-bit, but for now it's not needed. (Note, this limit does *not* prevent filesystems with 64-bit inode numbers from adding fscrypt support, since IV_INO_LBLK_* support is optional and is useful only on certain hardware.)

---
 fs/crypto/policy.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

index 2d73fd39ad96..b92f34523178 100644
@@ -192,10 +192,15 @@ static bool fscrypt_supported_v2_policy(const struct fscrypt_policy_v2 *policy,
 					  32, 32))
 		return false;
 
+	/*
+	 * IV_INO_LBLK_32 hashes the inode number, so in principle it can
+	 * support any ino_bits.  However, currently the inode number is gotten
+	 * from inode::i_ino which is 'unsigned long'.  So for now the
+	 * implementation limit is 32 bits.
+	 */
 	if ((policy->flags & FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32) &&
-	    /* This uses hashed inode numbers, so ino_bits doesn't matter. */
 	    !supported_iv_ino_lblk_policy(policy, inode, "IV_INO_LBLK_32",
-					  INT_MAX, 32))
+					  32, 32))
 		return false;
 
 	if (memchr_inv(policy->__reserved, 0, sizeof(policy->__reserved))) {

2.28.0.297.g1956fa8f8d-goog

Applied to fscrypt.git#master for 5.10.

- Eric