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

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Date:   Sun, 18 Nov 2018 16:31:22 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "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 1:30 PM, Christian Brauner <christian@...uner.io> wrote:
> On Sun, Nov 18, 2018 at 10:23:36PM +0100, Christian Brauner wrote:
>> On Sun, Nov 18, 2018 at 12:54:10PM -0800, Daniel Colascione wrote:
>> > On Sun, Nov 18, 2018 at 12:43 PM, Christian Brauner
>> > <christian@...uner.io> wrote:
>> > > On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:
>> > >>
>> > >>
>> > >> > 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.
>> > >
>> > > I can add a flag argument
>> > > int process_signal(int procfs_dfd, int signo, siginfo_t *info, int flags)
>> > > The way I see it process_signal() should be equivalent to kill(pid, signal) for now.
>> > > That is siginfo_t is cleared and set to:
>> > >
>> > > info.si_signo = sig;
>> > > info.si_errno = 0;
>> > > info.si_code = SI_USER;
>> > > info.si_pid = task_tgid_vnr(current);
>> > > info.si_uid = from_kuid_munged(current_user_ns(), current_uid());
>> >
>> > That makes sense. I just don't want to get into a situation where
>> > callers feel that they *have* to use the PID-based APIs to send a
>> > signal because process_kill doesn't offer some bit of functionality.
>>
>> Yeah.
>>
>> >
>> > Are you imagining something like requiring info t be NULL unless flags
>> > contains some "I have a siginfo_t" value?
>>
>> Well, I was actually thinking about something like:
>>
>> /**
>>  *  sys_process_signal - send a signal to a process trough a process file descriptor
>>  *  @fd: the file descriptor of the process
>>  *  @sig: signal to be sent
>>  *  @info: the signal info
>>  *  @flags: future flags to be passed
>>  */
>> SYSCALL_DEFINE4(process_signal, int, fd, int, sig, siginfo_t __user *, info,
>>               int, flags)
>> {
>>       struct pid *pid;
>>       struct fd *f;
>>       kernel_siginfo_t kinfo;
>>
>>       /* Do not allow users to pass garbage. */
>>       if (flags)
>>               return -EINVAL;
>>
>>       int ret = __copy_siginfo_from_user(sig, &kinfo, info);
>>       if (unlikely(ret))
>>               return ret;
>>
>>       /* For now, enforce that caller's creds are used. */
>>       kinfo.si_pid = task_tgid_vnr(current);
>>       kinfo.si_uid = from_kuid_munged(current_user_ns(), current_uid());

How about doing it this way? If info is NULL, act like kill(2);
otherwise, act like rt_sigqueueinfo(2).

(Not actual working or compiled code.)

SYSCALL_DEFINE4(process_signal, int, fd, int, sig, siginfo_t __user *, info,
              int, flags)
{
        struct fd f = { 0 };
        kernel_siginfo_t kinfo;
        int ret;

        /* Make API extension possible.  */
        ret = -EINVAL;
        if (flags)
                goto out;

        ret = -EBADF;
        f = fdget(fd);
        if (!f.file)
                goto out;

        ret = mumble_mumble_check_real_proc_file(f.file);
        if (ret)
                goto out;

        /* Act like kill(2) or rt_sigqueueinfo(2) depending on whether
         * the user gave us a siginfo structure.
         */
        if (info) {
                ret = __copy_siginfo_from_user(sig, &kinfo, info);
                if (ret)
                        goto out;
                /* Combine this logic with rt_sigqueueinfo(2) */
                ret = -EPERM;
                if ((info->si_code >= 0 || info->si_code == SI_TKILL) &&
                    (task_pid_vnr(current) != pid))
                        goto out;

        } else {
                /* Combine this logic with kill(2) */
                clear_siginfo(&kinfo);
                kinfo.si_signo = sig;
                kinfo.si_errno = 0;
                kinfo.si_code = SI_USER;
                kinfo.si_pid = task_tgid_vnr(current);
                kinfo.si_uid = from_kuid_munged(current_user_ns(),
current_uid());
        }

        ret = kill_pid_info(sig, &kinfo, proc_pid(file_inode(f.file)));

out:
        if (f.file)
                fput(f);
        return ret;
}