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Date: Sun, 2 May 2021 11:40:19 -0400
From: Felix Kyros Ang <felixk@...aven.com>
To: musl@...ts.openwall.com
Subject: Consider making issetugid compatible with Linux capabilities
Hello,
Musl implements the BSDs' issetugid syscall as a libc function (which
is awesome). Currently, musl's issetugid indicates that the
environment is tainted when it detects additional Linux capabilities
on the executable (set through setcap for example). This causes the
environment to be cleared when the executable is run, leading to some
program breakages. For instance, some libdbus functions use the
environment to identify the dbus session bus ID, and those programs
(linked to libdbus) will fail if Linux capabilities are added to it.
I wonder if it would be possible to tweak musl such that it does not
consider Linux capabilities a tainted privilege. Alternatively, it
could consider only the "highly overloaded" capabilities (such as
CAP_SYS_ADMIN) as tainted privileges. Here are my thoughts on why this
could be a good idea:
- The main purpose of capabilities is to add granular access control
mechanisms for sys administration in Linux, as opposed to the
"traditional" Unix black-and-white superuser model. The attack surface
is reduced, because programs may be granted only the elevated
permissions that they need to perform their function, as opposed to
the "full/complete" set of superuser privileges.
- By treating capabilities as equivalent to superuser privileges,
programs are forced to take up root privileges (e.g. they need to be
run as root or with sudo) in order to perform their intended
operations. This seems to defeat the purpose of adding granular
role-based access control mechanisms through capabilities in the first
place, and has the unfortunate consequence of re-opening the attack
surface again.
Some comparisons with the BSDs' behaviours seem to also support such a change:
- FreeBSD's issetugid checks if setuid and setgid bits are set, and if
the process's owner has been changed since program invocation which
could suggest a potential attempt at privilege escalation. Role-based
access controls are managed through the MAC mechanism, and this
appears to be compatible with issetugid (please correct me if I'm
wrong).
- OpenBSD keeps the old superuser model (making it important to be as
conservative as possible when calling issetugid). However, OpenBSD
uses pledge and unveil to *remove* privileges from superuser programs
(as opposed to grant additional privileges them on non-root
executables) to achieve the same effect.
issetugid seems to be particularly focused on process ownership
changes since program invocation (particular ownership changes from
lower to unqualified superuser privs - from user to root) as happens
with a setuid binary. It seems reasonable to consider a binary that
already has a Linux capability set to be invoking only that limited
subset of known privileges, and not be performing the equivalent of
taking on an "unknowable euid". If so, musl could explicitly decide
which capabilities it does or does not trust (like cap_sys_admin) when
returning whether the process is attempting to run as root or not.
Looking at the code, it looks like musl's issetugid function returns
the libc.secure value when called, and I couldn't find where
libc.secure's value is initialised. If someone could point me in the
right direction, I'll see if I can maybe send in a patch that proposes
a possible solution.
Thanks for your time. Looking forward to your thoughts.
Cheers,
Felix
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