linux-kernel - Re: [PATCH] procfs: Prevent unpriveleged processes accessing fdinfo

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Message-ID: <CAC_TJvfAsaM3AbK+P5PnYeNhKE-gXK6iL0WLURcJ0QSTGuYhwQ@mail.gmail.com>
Date:   Mon, 12 Jul 2021 12:45:51 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Koenig <christian.koenig@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Hridya Valsaraju <hridya@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] procfs: Prevent unpriveleged processes accessing fdinfo

On Sat, Jul 10, 2021 at 11:21 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Jul 8, 2021 at 8:57 AM Kalesh Singh <kaleshsingh@...gle.com> wrote:
> >
> > The file permissions on the fdinfo dir from were changed from
> > S_IRUSR|S_IXUSR to S_IRUGO|S_IXUGO, and a PTRACE_MODE_READ check was
> > added for opening the fdinfo files [1]. However, the ptrace permission
> > check was not added to the directory, allowing anyone to get the open FD
> > numbers by reading the fdinfo directory.
> >
> > Add the missing ptrace permission check for opening the fdinfo directory.
>
> The more I look at this, the more I feel like we should look at
> instead changing how "get_proc_task()" works.
>
> That's one of the core functions for /proc, and I wonder if we
> couldn't just make it refuse to look up a task that has gone through a
> suid execve() since the proc inode was opened.
>
> I don't think it's basically ever ok to open something for one thread,
> and then use it after the thread has gone through a suid thing.
>
> In fact, I wonder if we could make it even stricter, and go "any exec
> at all", but I think a suid exec might be the minimum we should do.
>
> Then the logic really becomes very simple: we did the permission
> checks at open time (like UNIX permission checks should be done), and
> "get_proc_task()" basically verifies that "yeah, that open-time
> decision is still valid".
>
> Wouldn't that make a lot of sense?

I think checking that the last open is after the last exec works, but
there are a few cases I’m not clear on:

Process A opens /proc/A/*/<file>. (Given it has the required
permissions - checked in open())

        Process A Start exec time = T1
        Proc inode open time /proc/A/*/<file>  = T2

T2 > T1: --> Process A can access /proc/A/*/<file> (FD 4)  -- OK


Process A does a fork and exec Process B

        Process B Start exec time = T3
        Proc inode open time /proc/A/*/<file>  = T2

T2 < T3: --> Process B can’t access /proc/A/*/<file> (by the copied FD 4) -- OK


Process B opens /proc/B/*/<file> (Given it has the required
permissions - checked in open())

        Process B Start exec time = T3
        Proc inode open time /proc/B/*/<file>  = T4.

T4 > T3: --> Process B can access /proc/B/*/<file> (FD 5)  -- OK


Process A opens /proc/A/*/<file> (Given it has the required
permissions - checked in open())

        Process A Start exec time = T1
        Proc inode open time /proc/A/*/<file>  = T5.

T5 > T1: --> Process A can access /proc/A/*/<file> (FD 5) -- OK

But,

        Process B Start exec time = T3
        Proc inode open time /proc/A/*/<file>  = T5.

T5 > T3: --> Process B can access /proc/A/*/<file> (by the copied FD
4) -- NOT OK


I think for the case above we could add a map to track the inode open
times per task at the cost of some added complexity.

For tracking the last exec times, I thought we could maybe reuse the
task_struct -> struct sched_entity se -> u64 exec_start /
sum_exec_runtime as indicators. These are relative to the task and set
to 0 on fork. But the inode open time needs to be comparable across
tasks in the case of a fork-exec as above. As I understand, we may
need a per-task field like last_exec_time, but I’m not sure we want to
incur the extra memory overhead for adding more fields to task_struct?

Please let me know if my understanding is not correct or if there is
something I overlooked here.

Thanks,
Kalesh

>
>              Linus