linux-kernel - Re: [PATCH] proc: allow killing processes via file descriptors

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Message-ID: <CAKOZuet4uzYjvNznfUvid2RH8kAuxteWWc26vLhJHKSfS6MjAA@mail.gmail.com>
Date:   Sun, 18 Nov 2018 12:32:56 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Jan Engelhardt <jengelh@...i.de>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors

On Sun, Nov 18, 2018 at 12:28 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> That is, I'm proposing an API that looks like this:
>>
>> int process_kill(int procfs_dfd, int signo, const union sigval value)
>>
>> If, later, process_kill were to *also* accept process-capability FDs,
>> nothing would break.
>
> Except that this makes it ambiguous to the caller as to whether their current creds are considered.  So it would need to be a different syscall or at least a flag.  Otherwise a lot of those nice theoretical properties go away.

Sure. A flag might make for better ergonomics.

>> Yes, that's what I have in mind. A siginfo_t is small enough that we
>> could just store it as a blob allocated off the procfs inode or
>> something like that without bothering with a shmfs file. You'd be able
>> to read(2) the exit status as many times as you wanted.
>
> I think that, if the syscall in question is read(2), then it should work *once* per struct file.  Otherwise running cat on the file would behave very oddly.

Why? The file pointer would work normally.

> Read and poll have the same problem as write: we can’t check caps in read or poll either.

Why not? Reading /proc/pid/stat does an access check today and
conditionally replaces the exit status with zero.