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Subject[PATCH] proc: allow killing processes via file descriptors
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With this patch an open() call on /proc/<pid> will give userspace a handle
to struct pid of the process associated with /proc/<pid>. This allows to
maintain a stable handle on a process.
I have been discussing various approaches extensively during technical
conferences this year culminating in a long argument with Eric at Linux
Plumbers. The general consensus was that having a handle on a process
will be something that is very simple and easy to maintain with the
option of being extensible via a more advanced api if the need arises. I
believe that this patch is the most simple, dumb, and therefore
maintainable solution.

The need for this has arisen in order to reliably kill a process without
running into issues of the pid being recycled as has been described in the
rejected patch [1]. To fulfill the need described in that patchset a new
ioctl() PROC_FD_SIGNAL is added. It can be used to send signals to a
process via a file descriptor:

int fd = open("/proc/1234", O_DIRECTORY | O_CLOEXEC);
ioctl(fd, PROC_FD_SIGNAL, SIGKILL);
close(fd);

Note, the stable handle will allow us to carefully extend this feature in
the future.

[1]: https://lkml.org/lkml/2018/10/30/118

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <christian@brauner.io>
---
fs/proc/base.c | 33 +++++++++++++++++++++++++++++++++
include/uapi/linux/procfd.h | 11 +++++++++++
2 files changed, 44 insertions(+)
create mode 100644 include/uapi/linux/procfd.h

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ce3465479447..dfde564a21eb 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -88,6 +88,7 @@
#include <linux/fs_struct.h>
#include <linux/slab.h>
#include <linux/sched/autogroup.h>
+#include <linux/sched/signal.h>
#include <linux/sched/mm.h>
#include <linux/sched/coredump.h>
#include <linux/sched/debug.h>
@@ -95,6 +96,7 @@
#include <linux/flex_array.h>
#include <linux/posix-timers.h>
#include <trace/events/oom.h>
+#include <uapi/linux/procfd.h>
#include "internal.h"
#include "fd.h"

@@ -3032,10 +3034,41 @@ static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
tgid_base_stuff, ARRAY_SIZE(tgid_base_stuff));
}

+static int proc_tgid_open(struct inode *inode, struct file *file)
+{
+ /* grab reference to struct pid */
+ file->private_data = get_pid(proc_pid(inode));
+ return 0;
+}
+
+static int proc_tgid_release(struct inode *inode, struct file *file)
+{
+ struct pid *pid = file->private_data;
+ /* drop reference to struct pid */
+ put_pid(pid);
+ return 0;
+}
+
+static long proc_tgid_ioctl(struct file *file, unsigned int cmd,
+ unsigned long arg)
+{
+ struct pid *pid = file->private_data;
+
+ switch (cmd) {
+ case PROC_FD_SIGNAL:
+ return kill_pid(pid, arg, 1);
+ default:
+ return -EINVAL;
+ }
+}
+
static const struct file_operations proc_tgid_base_operations = {
+ .open = proc_tgid_open,
.read = generic_read_dir,
.iterate_shared = proc_tgid_base_readdir,
.llseek = generic_file_llseek,
+ .release = proc_tgid_release,
+ .unlocked_ioctl = proc_tgid_ioctl,
};

static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
diff --git a/include/uapi/linux/procfd.h b/include/uapi/linux/procfd.h
new file mode 100644
index 000000000000..8e4c07a9f3a3
--- /dev/null
+++ b/include/uapi/linux/procfd.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef __LINUX_PROCFD_H
+#define __LINUX_PROCFD_H
+
+#include <linux/ioctl.h>
+
+/* Returns a file descriptor that refers to a struct pid */
+#define PROC_FD_SIGNAL _IOW('p', 1, __s32)
+
+#endif /* __LINUX_PROCFD_H */
+
--
2.19.1
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