fscrypt: avoid collisions when presenting long encrypted filenames

commit 6b06cdee81d68a8a829ad8e8d0f31d6836744af9 upstream.

When accessing an encrypted directory without the key, userspace must
operate on filenames derived from the ciphertext names, which contain
arbitrary bytes.  Since we must support filenames as long as NAME_MAX,
we can't always just base64-encode the ciphertext, since that may make
it too long.  Currently, this is solved by presenting long names in an
abbreviated form containing any needed filesystem-specific hashes (e.g.
to identify a directory block), then the last 16 bytes of ciphertext.
This needs to be sufficient to identify the actual name on lookup.

However, there is a bug.  It seems to have been assumed that due to the
use of a CBC (ciphertext block chaining)-based encryption mode, the last
16 bytes (i.e. the AES block size) of ciphertext would depend on the
full plaintext, preventing collisions.  However, we actually use CBC
with ciphertext stealing (CTS), which handles the last two blocks
specially, causing them to appear "flipped".  Thus, it's actually the
second-to-last block which depends on the full plaintext.

This caused long filenames that differ only near the end of their
plaintexts to, when observed without the key, point to the wrong inode
and be undeletable.  For example, with ext4:

    # echo pass | e4crypt add_key -p 16 edir/
    # seq -f "edir/abcdefghijklmnopqrstuvwxyz012345%.0f" 100000 | xargs touch
    # find edir/ -type f | xargs stat -c %i | sort | uniq | wc -l
    100000
    # sync
    # echo 3 > /proc/sys/vm/drop_caches
    # keyctl new_session
    # find edir/ -type f | xargs stat -c %i | sort | uniq | wc -l
    2004
    # rm -rf edir/
    rm: cannot remove 'edir/_A7nNFi3rhkEQlJ6P,hdzluhODKOeWx5V': Structure needs cleaning
    ...

To fix this, when presenting long encrypted filenames, encode the
second-to-last block of ciphertext rather than the last 16 bytes.

Although it would be nice to solve this without depending on a specific
encryption mode, that would mean doing a cryptographic hash like SHA-256
which would be much less efficient.  This way is sufficient for now, and
it's still compatible with encryption modes like HEH which are strong
pseudorandom permutations.  Also, changing the presented names is still
allowed at any time because they are only provided to allow applications
to do things like delete encrypted directories.  They're not designed to
be used to persistently identify files --- which would be hard to do
anyway, given that they're encrypted after all.

For ease of backports, this patch only makes the minimal fix to both
ext4 and f2fs.  It leaves ubifs as-is, since ubifs doesn't compare the
ciphertext block yet.  Follow-on patches will clean things up properly
and make the filesystems use a shared helper function.

Fixes: 5de0b4d0cd15 ("ext4 crypto: simplify and speed up filename encryption")
Reported-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
index 37b49894c762344841117b2a0042c5e9ec8b7140..15bf9c31a34d8f97ffd2f98a73562de0a47ae0ca 100644 (file)
--- a/fs/crypto/fname.c
+++ b/fs/crypto/fname.c
@@ -300,7 +300,7 @@ int fscrypt_fname_disk_to_usr(struct inode *inode,
        } else {
                memset(buf, 0, 8);
        }
-       memcpy(buf + 8, iname->name + iname->len - 16, 16);
+       memcpy(buf + 8, iname->name + ((iname->len - 17) & ~15), 16);
        oname->name[0] = '_';
        oname->len = 1 + digest_encode(buf, 24, oname->name + 1);
        return 0;

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 07e5e140577176e13db84089eaa048713ddb4797..eb2dea8287c16ea421f1f8a5a7dcd7ac04776843 100644 (file)
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1255,9 +1255,9 @@ static inline int ext4_match(struct ext4_filename *fname,
        if (unlikely(!name)) {
                if (fname->usr_fname->name[0] == '_') {
                        int ret;
-                       if (de->name_len < 16)
+                       if (de->name_len <= 32)
                                return 0;
-                       ret = memcmp(de->name + de->name_len - 16,
+                       ret = memcmp(de->name + ((de->name_len - 17) & ~15),
                                     fname->crypto_buf.name + 8, 16);
                        return (ret == 0) ? 1 : 0;
                }

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 589fe7c59fd19cf7f1c67b460b9c062888124061..96e9e7fa64dde434c478060e9e8e96813b5a5bb7 100644 (file)
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -139,8 +139,8 @@ struct f2fs_dir_entry *find_target_dentry(struct fscrypt_name *fname,
 #ifdef CONFIG_F2FS_FS_ENCRYPTION
                if (unlikely(!name->name)) {
                        if (fname->usr_fname->name[0] == '_') {
-                               if (de_name.len >= 16 &&
-                                       !memcmp(de_name.name + de_name.len - 16,
+                               if (de_name.len > 32 &&
+                                       !memcmp(de_name.name + ((de_name.len - 17) & ~15),
                                                fname->crypto_buf.name + 8, 16))
                                        goto found;
                                goto not_match;