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Date: Tue, 9 Jul 2019 13:58:37 +0000
From: Malte Kraus <malte.kraus@...e.com>
To: "oss-security@...ts.openwall.com" <oss-security@...ts.openwall.com>
Subject: Privileged File Access from Desktop Applications

With Wayland, it's no longer supported to run graphical applications as
root. The big desktop environments want to allow users to edit and
manage files through graphical applications (mostly text editors and
file managers). They have therefore implemented D-Bus services to
perform file I/O as root, authenticated through polkit.

We have reviewed various of these:
     a) a specialized D-Bus service for ktexteditor, the framework
        component powering the Kate editor
     b) gvfs-admin, which allows all gtk-enabled applications to access
        files as root
     c) the kauth helper for ioslave, which allows KDE applications to
        access files as root

As privileged file I/O is notoriously hard to get right, we found some
concrete security issues in them [1] [2], which the projects were glad
to fix. However, we believe there also some deeper design design
choices that also merit further discussion.

None of the solutions actually provided the user with the accessed path
or file operation in the polkit auth prompt. Users are confronted with
an unspecific request for privileges that they can only allow or deny
without knowing what exactly they are allowing. (This is unfortunately
a common theme, e.g. on KDE the framework is still missing support for
parameterizing polkit prompts.)

There's also a conceptual risk with the generic backends for file I/O:
unwitting applications that were written expecting to be run from an
unprivileged context can end up performing privileged file I/O
operations when supplied with such a URI/path. As a result, even when
the framework correctly implements all API guarantees for these
backends, applications are likely to be too careless with their
framework calls, resulting in situations where e.g. symlink attacks can
be performed. Of course due to the authentication requirements, user
interaction is involved in any such situation.

Another matter with gvfs-admin for example is what the desired
behaviour is when files are copied or moved between the 'normal' file
system and the virtual privileged file system - these operations are no
longer symmetric like they are traditionally: A rename(2) call is
either performed as root or not. But with a move of a file from file://
(unprivileged) to admin:// (privileged), there are two different
privilege levels involved. Since there is no equivalent to this
situation in the UNIX file system semantics, it is unclear who should
own the resulting file and what permissions it should have. Should file
ownership be preserved? Should the file be owned by root? If an
existing file is replaced, maybe the ownership and permissions of that
file should be preserved? How does a setgid bit on a directory affect
all this? What about higher-level flags in the framework like
G_FILE_COPY_ALL_METADATA and G_FILE_COPY_TARGET_DEFAULT_PERMS?

It's all quite underspecified, and we have the impression these
questions weren't fully thought through before the feature shipped.
Similar questions arise with the two KDE solutions, and some of the
bugs probably arose exactly because these questions are not answered
yet.


Best regards
Malte Kraus, Matthias Gerstner


[1] CVE-2018-10361 https://seclists.org/oss-sec/2018/q2/65
[2] CVE-2019-12447, CVE-2019-12448, CVE-2019-12449


-- 
Malte Kraus <malte.kraus@...e.com>
Security Engineer
PGP Key: 8AFC 3C58 6880 2DDD 4792  C3C2 FDBD 2984 D4C3 C2F0
SUSE Linux GmbH, GF: Felix Imend├Ârffer, Mary Higgins, Sri Rasiah, HRB
21284 (AG N├╝rnberg)

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