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SubjectRe: [RFC PATCH] ptrace: add PTRACE_GETFD request
On Thu, Dec 5, 2019 at 6:38 PM Jann Horn <jannh@google.com> wrote:
>
> On Fri, Dec 6, 2019 at 12:44 AM Sargun Dhillon <sargun@sargun.me> wrote:
> > PTRACE_GETFD is a generic ptrace API that allows the tracer to
> > get file descriptors from the traceee.
>
> typo: tracee
>
> > 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.
> [...]
> > +/* This gets a file descriptor from a running process. It doesn't require the
> > + * process to be stopped.
> > + */
> > +#define PTRACE_GETFD 0x420f
> [...]
> > +static int ptrace_getfd(struct task_struct *child, unsigned long fd)
>
> I'd make the "fd" parameter of this function an "unsigned int", given
> that that's also the argument type of fcheck_files().
>
> > +{
> > + 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);
>
> You're hardcoding the flags for the fd as 0, which means that there is
> no way for the caller to enable O_CLOEXEC on the fd in a way that is
> race-free against a concurrent execve(). If you can't easily plumb
> through an O_CLOEXEC flag from userspace to here, you should probably
> hardcode O_CLOEXEC here.
>
I thought about making addr used for flags. It seems a little weird, given the
name, but it'll do the job. Alternatively, it could be a point to an
options struct.
If we introduce options, one of the nice things we could add is add the ability
to cleanse the FD of certain information, like cgroups.

> > + if (ret >= 0)
> > + fd_install(ret, file);
> > +
> > + fput(file);
>
> Annoyingly, this isn't how fd_install() works. fd_install() has
> slightly weird semantics and consumes the reference passed to it, so
> this should be:
>
> if (ret >= 0)
> fd_install(ret, file);
> else
> fput(file);
>
> > +out:
> > + return ret;
> > +}

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