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SubjectRe: [PATCH v2 2/4] ptrace: add PTRACE_GETFD request to fetch file descriptors from tracees
On Mon, Dec 09, 2019 at 07:06:24AM +0000, Sargun Dhillon wrote:
> PTRACE_GETFD is a generic ptrace API that allows the tracer to
> get file descriptors from the tracee.
>
> One reason to use this is to allow sandboxers to take actions on file
> descriptors on the behalf of a tracee. For example, this can be
> combined with seccomp-bpf's user notification to ptrace on-demand and
> capture an fd without requiring the tracer to always be attached to
> the process. The tracer can then take a privileged action on behalf
> of the tracee, like binding a socket to a privileged port.
>
> It works whether or not the tracee is stopped. The only prior requirement
> is that the tracer is attached to the process via PTRACE_ATTACH or
> PTRACE_SEIZE. Stopping the process breaks certain runtimes that expect
> to be able to preempt syscalls (quickly). In addition, it is meant to be
> used in an on-demand fashion to avoid breaking debuggers.
>
> The ptrace call takes a pointer to ptrace_getfd_args in data, and the
> size of the structure in addr. There is an options field, which can
> be used to state whether the fd should be opened with CLOEXEC, or not.
> This options field may be extended 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.
>
> The requirement that the tracer has attached to the tracee prior to the
> capture of the file descriptor may be lifted at a later point.
>
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
> include/uapi/linux/ptrace.h | 15 +++++++++++++++
> kernel/ptrace.c | 35 +++++++++++++++++++++++++++++++++--
> 2 files changed, 48 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index a71b6e3b03eb..c84655bcc453 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -101,6 +101,21 @@ struct ptrace_syscall_info {
> };
> };
>
> +/*
> + * This gets a file descriptor from a process. It requires that the process
> + * has either been attached to. It does not require that the process is
> + * stopped.
> + */
> +#define PTRACE_GETFD 0x420f
> +
> +/* options to pass in to ptrace_getfd_args */
> +#define PTRACE_GETFD_O_CLOEXEC (1 << 0) /* open the fd with cloexec */

Hey Sargun,

Thanks for the patch!

Why not simply accept O_CLOEXEC as flag? If that's not possible for some
reason I'd say

#define PTRACE_GETFD_O_CLOEXEC O_CLOEXEC /* open the fd with cloexec */

is the right thing to do. This is fairly common:

include/uapi/linux/timerfd.h:#define TFD_CLOEXEC O_CLOEXEC
include/uapi/drm/drm.h:#define DRM_CLOEXEC O_CLOEXEC
include/linux/userfaultfd_k.h:#define UFFD_CLOEXEC O_CLOEXEC
include/linux/eventfd.h:#define EFD_CLOEXEC O_CLOEXEC
include/uapi/linux/eventpoll.h:#define EPOLL_CLOEXEC O_CLOEXEC
include/uapi/linux/inotify.h:/* For O_CLOEXEC and O_NONBLOCK */
include/uapi/linux/inotify.h:#define IN_CLOEXEC O_CLOEXEC
include/uapi/linux/mount.h:#define OPEN_TREE_CLOEXEC O_CLOEXEC /* Close the file on execve() */
You can also add a compile-time assert to ptrace like we did for
fs/namespace.c's OPEN_TREE_CLOEXEC:
BUILD_BUG_ON(OPEN_TREE_CLOEXEC != O_CLOEXEC);

And I'd remove the _O if you go with a separate flag, i.e.:

#define PTRACE_GETFD_CLOEXEC O_CLOEXEC /* open the fd with cloexec */
> +
> +struct ptrace_getfd_args {
> + __u32 fd; /* the tracee's file descriptor to get */
> + __u32 options;

Nit and I'm not set on it at all but "flags" might just be better.

> +} __attribute__((packed));

What's the benefit in using __attribute__((packed)) here? Seems to me that:

+struct ptrace_getfd_args {
+ __u32 fd; /* the tracee's file descriptor to get */
+ __u32 options;
+};
would just work fine.

> +
> /*
> * These values are stored in task->ptrace_message
> * by tracehook_report_syscall_* to describe the current syscall-stop.
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index cb9ddcc08119..8f619dceac6f 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -31,6 +31,7 @@
> #include <linux/cn_proc.h>
> #include <linux/compat.h>
> #include <linux/sched/signal.h>
> +#include <linux/fdtable.h>
>
> #include <asm/syscall.h> /* for syscall_get_* */
>
> @@ -994,6 +995,33 @@ ptrace_get_syscall_info(struct task_struct *child, unsigned long user_size,
> }
> #endif /* CONFIG_HAVE_ARCH_TRACEHOOK */
>
> +static int ptrace_getfd(struct task_struct *child, unsigned long user_size,
> + void __user *datavp)
> +{
> + struct ptrace_getfd_args args;
> + unsigned int fd_flags = 0;
> + struct file *file;
> + int ret;
> +
> + ret = copy_struct_from_user(&args, sizeof(args), datavp, user_size);
> + if (ret)
> + goto out;

Why is this goto out and not just return ret?

> + if ((args.options & ~(PTRACE_GETFD_O_CLOEXEC)) != 0)
> + return -EINVAL;
> + if (args.options & PTRACE_GETFD_O_CLOEXEC)
> + fd_flags &= O_CLOEXEC;
> + file = get_task_file(child, args.fd);
> + if (!file)
> + return -EBADF;
> + ret = get_unused_fd_flags(fd_flags);

Why isn't that whole thing just:

ret = get_unused_fd_flags(fd_flags & {PTRACE_GETFD_}O_CLOEXEC);
> + if (ret >= 0)
> + fd_install(ret, file);
> + else
> + fput(file);
> +out:
> + return ret;
> +}

So sm like:

static int ptrace_getfd(struct task_struct *child, unsigned long user_size,
void __user *datavp)
{
struct ptrace_getfd_args args;
unsigned int fd_flags = 0;
struct file *file;
int ret;
ret = copy_struct_from_user(&args, sizeof(args), datavp, user_size);
if (ret)
return ret;
if ((args.options & ~(PTRACE_GETFD_O_CLOEXEC)) != 0)
return -EINVAL;
file = get_task_file(child, args.fd);
if (!file)
return -EBADF;
/* PTRACE_GETFD_CLOEXEC == O_CLOEXEC */
ret = get_unused_fd_flags(fd_flags & PTRACE_GETFD_O_CLOEXEC);
if (ret >= 0)
fd_install(ret, file);
else
fput(file);
return ret;
}
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