linux-kernel - [RFC PATCH 0/1] security: add SECURE_KEEP_FSUID to preserve fsuid/fsgid across execve

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Date:   Thu, 13 Jun 2019 20:44:46 -0400
From:   Igor Lubashev <ilubashe@...mai.com>
To:     Serge Hallyn <serge@...lyn.com>, James Morris <jmorris@...ei.org>,
        <linux-security-module@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, Igor Lubashev <ilubashe@...mai.com>
Subject: [RFC PATCH 0/1] security: add SECURE_KEEP_FSUID to preserve fsuid/fsgid across execve

I've posted this in March but received no response. Reposting.

This patch introduces SECURE_KEEP_FSUID to allow fsuid/fsgid to be
preserved across execve. It is currently impossible to execve a
program such that effective and filesystem uid differ.

The need for this functionality arose from a desire to allow certain
non-privileged users to run perf. To do this, we install perf without
set-uid-root and have a set-uid-root wrapper decide who is allowed to
run perf (and with what arguments).

The wrapper must execve perf with real and effective root uid, because
perf and KASLR require this. However, that presently resets fsuid to
root, giving the user ability to read and overwrite any file owned by
root (perf report -i, perf record -o). Also, perf record will create
perf.data that cannot be deleted by the user.

We cannot reset /proc/sys/kernel/perf_event_paranoid to a permissive
level, since we must be selective which users have the permissions.

Of course, we could fix our problem by a patch to perf to allow
passing a username on the command line and having perf execute
setfsuid before opening files. However, perf is not the only program
that uses kernel features that require root uid/euid, so a general
solution that does not involve updating all such programs seems
warranted.

I will update man pages, if this patch is deemed a good idea.

Igor Lubashev (1):
  security: add SECURE_KEEP_FSUID to preserve fsuid/fsgid across execve

 include/uapi/linux/securebits.h | 10 +++++++++-
 security/commoncap.c            |  9 +++++++--
 2 files changed, 16 insertions(+), 3 deletions(-)

-- 
2.7.4