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SubjectRe: [PATCH] proc: allow killing processes via file descriptors
On Sun, Nov 18, 2018 at 04:31:22PM -0800, Daniel Colascione wrote:
> 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;
> }

Right, allowing to ass NULL might make sense. I had:

/**
* 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(procfd_kill, int, fd, int, sig, siginfo_t __user *, info, int, flags)
{
int ret;
struct pid *pid;
kernel_siginfo_t kinfo;
struct fd f;

/* Do not allow users to pass garbage. */
if (flags)
return -EINVAL;

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());

f = fdget_raw(fd);
if (!f.file)
return -EBADF;

ret = -EINVAL;
/* Is this a process file descriptor? */
if (!proc_is_procfd(f.file) || !d_is_dir(f.file->f_path.dentry))
goto err;

pid = f.file->private_data;
if (!pid)
goto err;

ret = -EPERM;
/*
* Not even root can pretend to send signals from the kernel.
* Nor can they impersonate a kill()/tgkill(), which adds source info.
*/
if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
(task_pid(current) != pid))
goto err;

ret = kill_pid_info(sig, &kinfo, pid);

err:
fdput(f);
return ret;
}

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