On Fri, Mar 26, 2021 at 9:05 AM Lorenz Bauer <lmb@xxxxxxxxxxxxxx> wrote:
>
> Invoking BPF_OBJ_GET on a pinned bpf_link checks the path access
> permissions based on file_flags, but the returned fd ignores flags.
> This means that any user can acquire a "read-write" fd for a pinned
> link with mode 0664 by invoking BPF_OBJ_GET with BPF_F_RDONLY in
> file_flags. The fd can be used to invoke BPF_LINK_DETACH, etc.
>
> Fix this by refusing non-O_RDWR flags in BPF_OBJ_GET. This works
> because OBJ_GET by default returns a read write mapping and libbpf
> doesn't expose a way to override this behaviour for programs
> and links.
>
> Fixes: 70ed506c3bbc ("bpf: Introduce pinnable bpf_link abstraction")
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---
LGTM.
Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> kernel/bpf/inode.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index 1576ff331ee4..dc56237d6960 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -547,7 +547,7 @@ int bpf_obj_get_user(const char __user *pathname, int flags)
> else if (type == BPF_TYPE_MAP)
> ret = bpf_map_new_fd(raw, f_flags);
> else if (type == BPF_TYPE_LINK)
> - ret = bpf_link_new_fd(raw);
> + ret = (f_flags != O_RDWR) ? -EINVAL : bpf_link_new_fd(raw);
> else
> return -ENOENT;
>
> --
> 2.27.0
>
