Re: [PATCH bpf-next 1/3] bpf: support O_PATH FDs in BPF_OBJ_PIN and BPF_OBJ_GET commands

Andrii Nakryiko
2023-05-16T18:02:58Z

On Tue, May 16, 2023 at 2:07 AM Christian Brauner <brauner@kernel.org> wrote:
>
> On Mon, May 15, 2023 at 05:13:46PM -0700, Andrii Nakryiko wrote:
> > Current UAPI of BPF_OBJ_PIN and BPF_OBJ_GET commands of bpf() syscall
> > forces users to specify pinning location as a string-based absolute or
> > relative (to current working directory) path. This has various
> > implications related to security (e.g., symlink-based attacks), forces
> > BPF FS to be exposed in the file system, which can cause races with
> > other applications.
> >
> > One of the feedbacks we got from folks working with containers heavily
> > was that inability to use purely FD-based location specification was an
> > unfortunate limitation and hindrance for BPF_OBJ_PIN and BPF_OBJ_GET
> > commands. This patch closes this oversight, adding path_fd field to
>
> Cool!
>
> > BPF_OBJ_PIN and BPF_OBJ_GET UAPI, following conventions established by
> > *at() syscalls for dirfd + pathname combinations.
> >
> > This now allows interesting possibilities like working with detached BPF
> > FS mount (e.g., to perform multiple pinnings without running a risk of
> > someone interfering with them), and generally making pinning/getting
> > more secure and not prone to any races and/or security attacks.
> >
> > This is demonstrated by a selftest added in subsequent patch that takes
> > advantage of new mount APIs (fsopen, fsconfig, fsmount) to demonstrate
> > creating detached BPF FS mount, pinning, and then getting BPF map out of
> > it, all while never exposing this private instance of BPF FS to outside
> > worlds.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> > include/linux/bpf.h | 4 ++--
> > include/uapi/linux/bpf.h | 5 +++++
> > kernel/bpf/inode.c | 16 ++++++++--------
> > kernel/bpf/syscall.c | 8 +++++---
> > tools/include/uapi/linux/bpf.h | 5 +++++
> > 5 files changed, 25 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 36e4b2d8cca2..f58895830ada 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -2077,8 +2077,8 @@ struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
> > struct bpf_link *bpf_link_get_from_fd(u32 ufd);
> > struct bpf_link *bpf_link_get_curr_or_next(u32 *id);
> >
> > -int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
> > -int bpf_obj_get_user(const char __user *pathname, int flags);
> > +int bpf_obj_pin_user(u32 ufd, int path_fd, const char __user *pathname);
> > +int bpf_obj_get_user(int path_fd, const char __user *pathname, int flags);
> >
> > #define BPF_ITER_FUNC_PREFIX "bpf_iter_"
> > #define DEFINE_BPF_ITER_FUNC(target, args...) \
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 1bb11a6ee667..db2870a52ce0 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -1420,6 +1420,11 @@ union bpf_attr {
> > __aligned_u64 pathname;
> > __u32 bpf_fd;
> > __u32 file_flags;
> > + /* same as dirfd in openat() syscall; see openat(2)
> > + * manpage for details of dirfd/path_fd and pathname semantics;
> > + * zero path_fd implies AT_FDCWD behavior
> > + */
> > + __u32 path_fd;
> > };
>
> So 0 is a valid file descriptor and can trivially be created and made to
> refer to any file. Is this a conscious decision to have a zero value
> imply AT_FDCWD and have you done this somewhere else in bpf already?
> Because that's contrary to how any file descriptor based apis work.
>
> How this is usually solved for extensible structs is to have a flag
> field that raises a flag to indicate that the fd fiel is set and thus 0
> can be used as a valid value.
>
> The way you're doing it right now is very counterintuitive to userspace
> and pretty much guaranteed to cause subtle bugs.

Yes, it's a very bpf()-specific convention we've settled on a while
ago. It allows a cleaner and simpler backwards compatibility story
without having to introduce new flags every single time. Most of BPF
UAPI by now dictates that (otherwise valid) FD 0 can't be used to pass
it to bpf() syscall. Most of the time users will be blissfully unaware
because libbpf and other BPF libraries are checking for fd == 0 and
dup()'ing them to avoid ever returning FD 0 to the user.

tl;dr, a conscious decision consistent with the rest of BPF UAPI. It
is a bpf() peculiarity, yes.