> On Dec 5, 2019, at 3:44 PM, Sargun Dhillon <sargun@xxxxxxxxx> 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. Can you document the circumstances under which you can call this? Does it require that you be attached as a tracer? Does the tracee need to be stopped? Does it work equivalently if the tracee is ptrace-stopped vs stopped by seccomp? If the tracee is running and is single-threaded, is the locking correct? > > Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx> > --- > 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 >