Commit f000223c authored Jul 21, 2020 by Eric Biggers

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: b103fb76 ("fscrypt: add support for IV_INO_LBLK_64 policies")
Fixes: e3b1078b ("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>

parent 1d6217a4

fs/crypto/policy.c

+14 −0

Original line number	Diff line number	Diff line
		@@ -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.

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