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

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Message-ID: <alpine.LRH.2.21.1906141338350.7150@namei.org>
Date:   Fri, 14 Jun 2019 13:38:53 +1000 (AEST)
From:   James Morris <jmorris@...ei.org>
To:     Igor Lubashev <ilubashe@...mai.com>
cc:     Serge Hallyn <serge@...lyn.com>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
        Al Viro <viro@....linux.org.uk>
Subject: Re: [RFC PATCH 0/1] security: add SECURE_KEEP_FSUID to preserve
 fsuid/fsgid across execve

[Adding David and Al]


On Thu, 13 Jun 2019, Igor Lubashev wrote:

> 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(-)
> 
> 

-- 
James Morris
<jmorris@...ei.org>