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authorDaniel Rosenberg <drosen@google.com>2020-01-20 14:31:57 -0800
committerEric Biggers <ebiggers@google.com>2020-01-22 14:49:55 -0800
commitaa408f835d025a839033988d3f5a2866314414ef (patch)
treed16ad29e5084aefbded75342d81ed35abb9584ab /fs/crypto/hooks.c
parent6e1918cfb263acacd3fc9239127732b69de64695 (diff)
fscrypt: derive dirhash key for casefolded directories
When we allow indexed directories to use both encryption and casefolding, for the dirhash we can't just hash the ciphertext filenames that are stored on-disk (as is done currently) because the dirhash must be case insensitive, but the stored names are case-preserving. Nor can we hash the plaintext names with an unkeyed hash (or a hash keyed with a value stored on-disk like ext4's s_hash_seed), since that would leak information about the names that encryption is meant to protect. Instead, if we can accept a dirhash that's only computable when the fscrypt key is available, we can hash the plaintext names with a keyed hash using a secret key derived from the directory's fscrypt master key. We'll use SipHash-2-4 for this purpose. Prepare for this by deriving a SipHash key for each casefolded encrypted directory. Make sure to handle deriving the key not only when setting up the directory's fscrypt_info, but also in the case where the casefold flag is enabled after the fscrypt_info was already set up. (We could just always derive the key regardless of casefolding, but that would introduce unnecessary overhead for people not using casefolding.) Signed-off-by: Daniel Rosenberg <drosen@google.com> [EB: improved commit message, updated fscrypt.rst, squashed with change that avoids unnecessarily deriving the key, and many other cleanups] Link: https://lore.kernel.org/r/20200120223201.241390-3-ebiggers@kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com>
Diffstat (limited to 'fs/crypto/hooks.c')
-rw-r--r--fs/crypto/hooks.c16
1 files changed, 16 insertions, 0 deletions
diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index fd4e5ae21077..5ef861742921 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -5,6 +5,8 @@
* Encryption hooks for higher-level filesystem operations.
*/
+#include <linux/key.h>
+
#include "fscrypt_private.h"
/**
@@ -137,8 +139,14 @@ int fscrypt_prepare_setflags(struct inode *inode,
unsigned int oldflags, unsigned int flags)
{
struct fscrypt_info *ci;
+ struct fscrypt_master_key *mk;
int err;
+ /*
+ * When the CASEFOLD flag is set on an encrypted directory, we must
+ * derive the secret key needed for the dirhash. This is only possible
+ * if the directory uses a v2 encryption policy.
+ */
if (IS_ENCRYPTED(inode) && (flags & ~oldflags & FS_CASEFOLD_FL)) {
err = fscrypt_require_key(inode);
if (err)
@@ -146,6 +154,14 @@ int fscrypt_prepare_setflags(struct inode *inode,
ci = inode->i_crypt_info;
if (ci->ci_policy.version != FSCRYPT_POLICY_V2)
return -EINVAL;
+ mk = ci->ci_master_key->payload.data[0];
+ down_read(&mk->mk_secret_sem);
+ if (is_master_key_secret_present(&mk->mk_secret))
+ err = fscrypt_derive_dirhash_key(ci, mk);
+ else
+ err = -ENOKEY;
+ up_read(&mk->mk_secret_sem);
+ return err;
}
return 0;
}