Re: [RFC PATCH] ptrace: add PTRACE_GETFD request

[Christian Brauner <christian.brauner@ubuntu.com>]

[+ Oleg, the maintainer. This needs to see his review before anything
 can happen to this series.]

On Thu, Dec 05, 2019 at 11:44:53PM +0000, Sargun Dhillon wrote:
> PTRACE_GETFD is a generic ptrace API that allows the tracer to
> get file descriptors from the traceee.
> 
> The primary reason to use this syscall is to allow sandboxers to
> take action on an FD on behalf of the tracee. For example, this
> can be combined with seccomp's user notification feature to extract
> a file descriptor and call privileged syscalls, like binding
> a socket to a privileged port.
> 
> Signed-off-by: Sargun Dhillon <sargun@sargun.me>
> ---
>  include/uapi/linux/ptrace.h |  5 +++++
>  kernel/ptrace.c             | 39 +++++++++++++++++++++++++++++++++++--
>  2 files changed, 42 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
> index a71b6e3b03eb..2b69f759826a 100644
> --- a/include/uapi/linux/ptrace.h
> +++ b/include/uapi/linux/ptrace.h
> @@ -101,6 +101,11 @@ struct ptrace_syscall_info {
>  	};
>  };
>  
> +/* This gets a file descriptor from a running process. It doesn't require the
> + * process to be stopped.
> + */
> +#define PTRACE_GETFD	0x420f
> +
>  /*
>   * 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..a1d7b289fe8e 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 +995,37 @@ 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 fd)
> +{
> +	struct files_struct *files;
> +	struct file *file;
> +	int ret = 0;
> +
> +	files = get_files_struct(child);
> +	if (!files)
> +		return -ENOENT;
> +
> +	spin_lock(&files->file_lock);
> +	file = fcheck_files(files, fd);
> +	if (!file)
> +		ret = -EBADF;
> +	else
> +		get_file(file);
> +	spin_unlock(&files->file_lock);
> +	put_files_struct(files);
> +
> +	if (ret)
> +		goto out;
> +
> +	ret = get_unused_fd_flags(0);
> +	if (ret >= 0)
> +		fd_install(ret, file);
> +
> +	fput(file);
> +out:
> +	return ret;
> +}
> +
>  int ptrace_request(struct task_struct *child, long request,
>  		   unsigned long addr, unsigned long data)
>  {
> @@ -1222,7 +1254,9 @@ int ptrace_request(struct task_struct *child, long request,
>  	case PTRACE_SECCOMP_GET_METADATA:
>  		ret = seccomp_get_metadata(child, addr, datavp);
>  		break;
> -
> +	case PTRACE_GETFD:
> +		ret = ptrace_getfd(child, data);
> +		break;
>  	default:
>  		break;
>  	}
> @@ -1265,7 +1299,8 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
>  	}
>  
>  	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
> -				  request == PTRACE_INTERRUPT);
> +				  request == PTRACE_INTERRUPT ||
> +				  request == PTRACE_GETFD);
>  	if (ret < 0)
>  		goto out_put_task_struct;
>  
> -- 
> 2.20.1
>