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

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Message-Id: <608F2959-800D-46EE-A7CD-8C972ACD2F02@amacapital.net>
Date:   Sun, 18 Nov 2018 13:28:41 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Daniel Colascione <dancol@...gle.com>
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 Nov 18, 2018, at 12:44 PM, Daniel Colascione <dancol@...gle.com> 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.

> 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.

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