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authorEric Biggers <ebiggers@google.com>2017-04-24 10:00:09 -0700
committerTheodore Ts'o <tytso@mit.edu>2017-05-04 11:44:36 -0400
commit6b06cdee81d68a8a829ad8e8d0f31d6836744af9 (patch)
tree4ccb45ef816cf493c8222dd690b9a76701087abf /fs/ext4/namei.c
parent6332cd32c8290a80e929fc044dc5bdba77396e33 (diff)
downloadlinux-stericsson-6b06cdee81d68a8a829ad8e8d0f31d6836744af9.tar.gz
fscrypt: avoid collisions when presenting long encrypted filenames
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> Cc: stable@vger.kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Diffstat (limited to 'fs/ext4/namei.c')
-rw-r--r--fs/ext4/namei.c4
1 files changed, 2 insertions, 2 deletions
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index df3f9d799e2d..2d866e4a9e7a 100644
--- 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;
}