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SubjectRe: [PATCH] proc: allow killing processes via file descriptors
On Sun, Nov 18, 2018 at 1:30 PM, Christian Brauner <christian@brauner.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@brauner.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@google.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;
}

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