From: Lorenz Bauer lmb@cloudflare.com
commit 25fc94b2f02d832fa8e29419699dcc20b0b05c6a upstream.
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@cloudflare.com Signed-off-by: Alexei Starovoitov ast@kernel.org Acked-by: Andrii Nakryiko andrii@kernel.org Acked-by: Daniel Borkmann daniel@iogearbox.net Link: https://lore.kernel.org/bpf/20210326160501.46234-1-lmb@cloudflare.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/inode.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/bpf/inode.c +++ b/kernel/bpf/inode.c @@ -546,7 +546,7 @@ int bpf_obj_get_user(const char __user * 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;