Comment 111 for bug 745836

Colin Ian King (colin-king) wrote :

SRU justification for Lucid:

Impact:

The ECRYPTFS_NEW_FILE crypt_stat flag is set upon creation of a new
eCryptfs file. When the flag is set, eCryptfs reads directly from the
lower filesystem when bringing a page up to date. This means that no
offset translation (for the eCryptfs file metadata in the lower file)
and no decryption is performed. The flag is cleared just before the
first write is completed (at the beginning of ecryptfs_write_begin()).

It was discovered that if a new file was created and then extended with
truncate, the ECRYPTFS_NEW_FILE flag was not cleared. If pages
corresponding to this file are ever reclaimed, any subsequent reads
would result in userspace seeing eCryptfs file metadata and encrypted
file contents instead of the expected decrypted file contents.

Data corruption is possible if the file is written to before the
eCryptfs directory is unmounted. The data written will be copied into
pages which have been read directly from the lower file rather than
zeroed pages, as would be expected after extending the file with
truncate.

Fix: Clear the ECRYPTFS_NEW_FILE flags if set. Fix was originally from
Tyler Hicks and needed a little massaging to apply for the current Lucid,
see https://launchpadlibrarian.net/82254993/0001-eCryptfs-Clear-ECRYPTFS_NEW_FILE-flag-during-truncat.patch

Testcase:

foo && truncate -s 4096 foo && sync && echo 1 | sudo tee /proc/sys/vm/drop_caches && hexdump -C foo

and hexdump should show a file filled with zeroes. Without the fix the file
is full of garbage, whereas with the fix the file is full of zeros as
expected.