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SubjectRe: [PATCH v3 2/4] pid: Add PIDFD_IOCTL_GETFD to fetch file descriptors from processes
[Cc Arnd since he fiddled with ioctl()s quite a bit recently.]

On Tue, Dec 17, 2019 at 01:00:04AM +0000, Sargun Dhillon wrote:
> This adds an ioctl which allows file descriptors to be extracted
> from processes based on their pidfd.
>
> One reason to use this is to allow sandboxers to take actions on file
> descriptors on the behalf of another process. For example, this can be
> combined with seccomp-bpf's user notification to do on-demand fd
> extraction and take privileged actions. For example, it can be used
> to bind a socket to a privileged port. This is similar to ptrace, and
> using ptrace parasitic code injection to extract a file descriptor from a
> process, but without breaking debuggers, or paying the ptrace overhead
> cost.
>
> You must have the ability to ptrace the process in order to extract any
> file descriptors from it. ptrace can already be used to extract file
> descriptors based on parasitic code injections, so the permissions
> model is aligned.
>
> The ioctl takes a pointer to pidfd_getfd_args. pidfd_getfd_args contains
> a size, which allows for gradual evolution of the API. There is an options
> field, which can be used to state whether the fd should be opened with
> CLOEXEC, or not. An additional options field may be added in the future
> to include the ability to clear cgroup information about the file
> descriptor at a later point. If the structure is from a newer kernel, and
> includes members which make it larger than the structure that's known to
> this kernel version, E2BIG will be returned.
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
> Documentation/ioctl/ioctl-number.rst | 1 +
> include/linux/pid.h | 1 +
> include/uapi/linux/pid.h | 26 ++++++++++
> kernel/fork.c | 72 ++++++++++++++++++++++++++++
> 4 files changed, 100 insertions(+)
> create mode 100644 include/uapi/linux/pid.h
>
> diff --git a/Documentation/ioctl/ioctl-number.rst b/Documentation/ioctl/ioctl-number.rst
> index bef79cd4c6b4..be2efb93acd1 100644
> --- a/Documentation/ioctl/ioctl-number.rst
> +++ b/Documentation/ioctl/ioctl-number.rst
> @@ -272,6 +272,7 @@ Code Seq# Include File Comments
> <mailto:tim@cyberelk.net>
> 'p' A1-A5 linux/pps.h LinuxPPS
> <mailto:giometti@linux.it>
> +'p' B0-CF uapi/linux/pid.h
> 'q' 00-1F linux/serio.h
> 'q' 80-FF linux/telephony.h Internet PhoneJACK, Internet LineJACK
> linux/ixjuser.h <http://web.archive.org/web/%2A/http://www.quicknet.net>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index 9645b1194c98..65f1a73040c9 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -5,6 +5,7 @@
> #include <linux/rculist.h>
> #include <linux/wait.h>
> #include <linux/refcount.h>
> +#include <uapi/linux/pid.h>

That should be pidfd.h and the resulting new file be placed under the
pidfd entry in maintainers:
+F: include/uapi/linux/pidfd.h

>
> enum pid_type
> {
> diff --git a/include/uapi/linux/pid.h b/include/uapi/linux/pid.h
> new file mode 100644
> index 000000000000..4ec02ed8b39a
> --- /dev/null
> +++ b/include/uapi/linux/pid.h
> @@ -0,0 +1,26 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef _UAPI_LINUX_PID_H
> +#define _UAPI_LINUX_PID_H
> +
> +#include <linux/types.h>
> +#include <linux/ioctl.h>
> +
> +/* options to pass in to pidfd_getfd_args flags */
> +#define PIDFD_GETFD_CLOEXEC (1 << 0) /* open the fd with cloexec */
Please, make them cloexec by default unless there's a very good reason
not to.

> +
> +struct pidfd_getfd_args {
> + __u32 size; /* sizeof(pidfd_getfd_args) */
> + __u32 fd; /* the tracee's file descriptor to get */
> + __u32 flags;
> +};

I think you want to either want to pad this

+struct pidfd_getfd_args {
+ __u32 size; /* sizeof(pidfd_getfd_args) */
+ __u32 fd; /* the tracee's file descriptor to get */
+ __u32 flags;
__u32 reserved;
+};
or use __aligned_u64 everywhere which I'd personally prefer instead of
this manual padding everywhere.

> +
> +#define PIDFD_IOC_MAGIC 'p'
> +#define PIDFD_IO(nr) _IO(PIDFD_IOC_MAGIC, nr)
> +#define PIDFD_IOR(nr, type) _IOR(PIDFD_IOC_MAGIC, nr, type)
> +#define PIDFD_IOW(nr, type) _IOW(PIDFD_IOC_MAGIC, nr, type)
> +#define PIDFD_IOWR(nr, type) _IOWR(PIDFD_IOC_MAGIC, nr, type)
> +
> +#define PIDFD_IOCTL_GETFD PIDFD_IOWR(0xb0, \
> + struct pidfd_getfd_args)
> +
> +#endif /* _UAPI_LINUX_PID_H */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 6cabc124378c..d9971e664e82 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1726,9 +1726,81 @@ static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
> return poll_flags;
> }
>
> +static long pidfd_getfd(struct pid *pid, struct pidfd_getfd_args __user *buf)
> +{
> + struct pidfd_getfd_args args;
> + unsigned int fd_flags = 0;
> + struct task_struct *task;
> + struct file *file;
> + u32 user_size;
> + int ret, fd;
> +
> + ret = get_user(user_size, &buf->size);
> + if (ret)
> + return ret;
> +
> + ret = copy_struct_from_user(&args, sizeof(args), buf, user_size);
> + if (ret)
> + return ret;
> + if ((args.flags & ~(PIDFD_GETFD_CLOEXEC)) != 0)
> + return -EINVAL;

Nit: It's more common - especially in this file - to do

if (args.flags & ~PIDFD_GETFD_CLOEXEC)
return -EINVAL;
> + if (args.flags & PIDFD_GETFD_CLOEXEC)
> + fd_flags |= O_CLOEXEC;
> +
> + task = get_pid_task(pid, PIDTYPE_PID);
> + if (!task)
> + return -ESRCH;

\n

> + ret = -EPERM;
> + if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
> + goto out;

\n

Please don't pre-set errors unless they are used by multiple exit paths.
if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
ret = -EPERM;
goto out;
}
> + ret = -EBADF;
> + file = fget_task(task, args.fd);
> + if (!file)
> + goto out;

Same.

> +
> + fd = get_unused_fd_flags(fd_flags);
> + if (fd < 0) {
> + ret = fd;
> + goto out_put_file;
> + }

\n

> + /*
> + * security_file_receive must come last since it may have side effects
> + * and cannot be reversed.
> + */
> + ret = security_file_receive(file);
> + if (ret)
> + goto out_put_fd;
> +
> + fd_install(fd, file);
> + put_task_struct(task);
> + return fd;
> +
> +out_put_fd:
> + put_unused_fd(fd);
> +out_put_file:
> + fput(file);
> +out:
> + put_task_struct(task);
> + return ret;
> +}
> +
> +static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> +{
> + struct pid *pid = file->private_data;
> + void __user *buf = (void __user *)arg;
> +
> + switch (cmd) {
> + case PIDFD_IOCTL_GETFD:
> + return pidfd_getfd(pid, buf);
> + default:
> + return -EINVAL;
> + }
> +}
> +
> const struct file_operations pidfd_fops = {
> .release = pidfd_release,
> .poll = pidfd_poll,
> + .unlocked_ioctl = pidfd_ioctl,
> #ifdef CONFIG_PROC_FS
> .show_fdinfo = pidfd_show_fdinfo,
> #endif
> --
> 2.20.1
>

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