Re: [PATCH] procfs: Prevent unpriveleged processes accessing fdinfo

[ebiederm@xmission.com (Eric W. Biederman)]

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Thu, Jul 8, 2021 at 8:57 AM Kalesh Singh <kaleshsingh@google.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.

The practical implementation that I can see is to add a
exec_id attribute into the proc inode and to modify proc_pid_make_inode
to take a new exec_id parameter.

There are some directories like /proc/PPP/, /proc/PPP/task/TTT/,
/proc/PPP/net where it is both safe and appropriate to allow caching the
reference over a suid exec.

To handle that I would have a flag somewhere (possibly a special exec_id
value) that indicates we don't care about the exec id.

Once get_proc_task is taught to handle both cases and the appropriate
exec_id is passed to proc_pid_make_inode proc_pid_invalidate works
automatically.  So I think that is all we really need to do.

> 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?

Roughly.  I want to use reuse exec_id but that seems a bit strong for
have the permissions changed.  Checking ->cred is too sensitive.
So it is a bit fiddly to get right.

Limiting this to suid-exec (and equivalent) seems like the proper
filter, because it is when the permissions have fundamentally changed.

I just don't think this should be blanket for everything that uses
get_prock_task.

Eric