linux-kernel - Re: [PATCH RFC v2 5/6] proc: instantiate only pids that we can ptrace on 'limit

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Date:   Tue, 2 May 2017 16:00:41 +0200
From:   Djalal Harouni <tixxdz@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Dongsu Park <dpark@...teo.net>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH RFC v2 5/6] proc: instantiate only pids that we can ptrace
 on 'limit_pids=1' mount option

Hello Andy,

(Sorry for my late response)

On Thu, Apr 27, 2017 at 12:09 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Tue, Apr 25, 2017 at 5:23 AM, Djalal Harouni <tixxdz@...il.com> wrote:
>> If "limit_pids=1" mount option is set then do not instantiate pids that
>> we can not ptrace. "limit_pids=1" means that procfs should only contain
>> pids that the caller can ptrace.
>>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Andy Lutomirski <luto@...nel.org>
>> Signed-off-by: Djalal Harouni <tixxdz@...il.com>
>> ---
>>  fs/proc/base.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 2e0f661..a663284 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -3149,6 +3149,7 @@ struct dentry *proc_pid_lookup(struct inode *dir, struct dentry * dentry, unsign
>>         unsigned tgid;
>>         struct proc_fs_info *fs_info = proc_sb(dir->i_sb);
>>         struct pid_namespace *ns = fs_info->pid_ns;
>> +       int limit_pids = proc_fs_limit_pids(fs_info);
>
> Shouldn't the addition of proc_fs_limit_pids() be in this patch?

Actually I think this patch should be part of proc_fs_limit_pids()
since it will cover paths that are not handled by inode ->permission()
checks. But I will review this patch ad discussion "devpts: Make each
mount of devpts an independent filesystem" [1] and try to get the
rational about it, if doing permission like checks in lookups is
related, or if it should be done in the first place...

The other thing that I didn't have time to check is standardizing on
returned error codes: I prefer to always return -ENOENT, instead of
-EPERM or sometimes -ENOENT.

> Also, can we name it something self-documented?
> "ptraceable_pids_only=1", perhaps?  Or even pids=ptraceable (as
> opposed to pids=all or maybe other choices in the future)?

Yes, I will update.

[1] https://patchwork.kernel.org/patch/9150781/

-- 
tixxdz