linux-kernel - Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF

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Date:   Fri, 24 Feb 2023 16:27:19 -0800
From:   Andrei Vagin <avagin@...il.com>
To:     Giuseppe Scrivano <gscrivan@...hat.com>
Cc:     Colin Walters <walters@...bum.org>,
        Christian Brauner <brauner@...nel.org>,
        Aleksa Sarai <cyphar@...har.com>, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>, bristot@...hat.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alexander Larsson <alexl@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, bmasney@...hat.com
Subject: Re: [PATCH v3 1/2] exec: add PR_HIDE_SELF_EXE prctl

On Mon, Jan 30, 2023 at 11:06:02AM +0100, Christian Brauner wrote:
> On Mon, Jan 30, 2023 at 10:53:31AM +0100, Christian Brauner wrote:
> > On Sun, Jan 29, 2023 at 01:12:45PM -0500, Colin Walters wrote:
> > > 
> > > 
> > > On Sun, Jan 29, 2023, at 11:58 AM, Christian Brauner wrote:
> > > > On Sun, Jan 29, 2023 at 08:59:32AM -0500, Colin Walters wrote:
> > > >> 
> > > >> 
> > > >> On Wed, Jan 25, 2023, at 11:30 AM, Giuseppe Scrivano wrote:
> > > >> > 
> > > >> > After reading some comments on the LWN.net article, I wonder if
> > > >> > PR_HIDE_SELF_EXE should apply to CAP_SYS_ADMIN in the initial user
> > > >> > namespace or if in this case root should keep the privilege to inspect
> > > >> > the binary of a process.  If a container runs with that many privileges
> > > >> > then it has already other ways to damage the host anyway.
> > > >> 
> > > >> Right, that's what I was trying to express with the "make it work the same as map_files".  Hiding the entry entirely even for initial-namespace-root (real root) seems like it's going to potentially confuse profiling/tracing/debugging tools for no good reason.
> > > >
> > > > If this can be circumvented via CAP_SYS_ADMIN 
> > > 
> > > To be clear, I'm proposing CAP_SYS_ADMIN in the current user namespace at the time of the prctl().  (Or if keeping around a reference just for this is too problematic, perhaps hardcoding to the init ns)
> > 
> > Oh no, I fully understand. The point was that the userspace fix protects
> > even against attackers with CAP_SYS_ADMIN in init_user_ns. And that was
> > important back then and is still relevant today for some workloads.
> > 
> > For unprivileged containers where host and container are separate by a
> > meaningful user namespace boundary this whole mitigation is irrelevant
> > as the binary can't be overwritten.
> > 
> > > 
> > > A process with CAP_SYS_ADMIN in a child namespace would still not be able to read the binary.
> > > 
> > > > then this mitigation
> > > > becomes immediately way less interesting because the userspace
> > > > mitigation we came up with protects against CAP_SYS_ADMIN as well
> > > > without any regression risk. 
> > > 
> > > The userspace mitigation here being "clone self to memfd"?  But that's a sufficiently ugly workaround that it's created new problems; see https://lwn.net/Articles/918106/
> > 
> > But this is a problem with the memfd api not with the fix. Following the
> > thread the ability to create executable memfds will stay around. As it
> > should be given how long this has been supported. And they have backward
> > compatibility in mind which is great.
> 
> Following up from yesterday's promise to check with the criu org I'm
> part of: this is going to break criu unforunately as it dumps (and
> restores) /proc/self/exe. Even with an escape hatch we'd still risk
> breaking it. Whereas again, the memfd solution doesn't cause those
> issues.
> 
> Don't get me wrong it's pretty obvious that I was pretty supportive of
> this fix especially because it looked rather simple but this is turning
> out to be less simple than we tought. I don't think that this is worth
> it given the functioning fixes we already have.

btw: can we use PR_SET_MM_EXE_FILE or PR_SET_MM_MAP (prctl_map.exe_fd) to
set a dummy exe. Will it have the required effect?

> 
> The good thing is that - even if it will take a longer - that Aleksa's
> patchset will provide a more general solution by making it possible for
> runc/crun/lxc to open the target binary with a restricted upgrade mask
> making it impossible to open the binary read-write again. This won't
> break criu and will fix this issue and is generally useful.