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Date: Tue, 22 Jul 2014 17:35:07 -0500
From: Tyler Hicks <tyhicks@...onical.com>
To: oss-security@...ts.openwall.com
Cc: Dustin Kirkland <kirkland@...ntu.com>
Subject: Re: ecryptfs-setup-private nitpick
Hi Raphael!
On 2014-07-22 14:00:03, Raphael Geissert wrote:
> Hi,
>
> Taking a look at ecryptfs-utils 103's ecryptfs-setup-private, there is a bit
> of code that writes the mount pass to a file in /dev/shm hoping to "keep it
> from leaking to the hard-drive":
>
> 8<-------->8
> # This will be wrapped by pam_ecryptfs's chauthtok as soon as the
> user
> # chooses a password. Until that happens (hopefully soon), standard
> # file permissions (600) are all that's protecting it. Write it to
> # ramdisk, to keep it from leaking to the hard-drive.
> temp=`mktemp /dev/shm/.ecryptfs-XXXXXX`
> printf "%s" "$MOUNTPASS" > "$temp"
> mv -f -T "$temp" "/dev/shm/.ecryptfs-$USER" || error "Could not
> create passphrase file"
> 8<-------->8
>
> Fastforward to 2014 and /dev/shm is, well, not a ramfs/ramdisk:
>
> /dev/shm -> /run/shm, which is a tmpfs at least on Debian.
>
> And as clearly stated by Documentation/filesystems/tmpfs.txt:
> "If you compare it to ramfs (which was the template to create tmpfs)
> you gain swapping and limit checking."
>
>
> So in the hope of avoiding a persistent storage the mount pass is written to
> a file in a tmpfs that can be swapped to... disk.
I consider encrypted swap to be a prerequisite to enabling any
disk/file encryption solution. Ubuntu sets up encrypted swap when the
user selects to encrypt their home directory from the installer.
Unfortunately, the ecryptfs-setup-private man page doesn't recommend
encrypting your swap but ecryptfs-utils ships a script called
ecryptfs-setup-swap that enables encrypted swap.
Ignoring the encrypted swap argument, ecryptfs-setup-private shouldn't
be storing the plaintext mount passphrase in a manner that is swappable.
I think POSIX shared memory segments should provide the persistence and
pinnable memory (SHM_LOCKED) needed.
Either Dustin (cc'ed) or I will make this improvement. Thanks for the
feedback!
Tyler
>
> The file is left on /dev/shm until pam_ecryptfs actually wraps it with the
> login pass.
>
> Cheers,
> --
> Raphael Geissert - Debian Developer
> www.debian.org - get.debian.net
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