bpf: fail BPF_TOKEN_CREATE if no delegation option was set on BPF FS

It's quite confusing in practice when it's possible to successfully
create a BPF token from BPF FS that didn't have any of delegate_xxx
mount options set up. While it's not wrong, it's actually more
meaningful to reject BPF_TOKEN_CREATE with specific error code (-ENOENT)
to let user-space know that no token delegation is setup up.

So, instead of creating empty BPF token that will be always ignored
because it doesn't have any of the allow_xxx bits set, reject it with
-ENOENT. If we ever need empty BPF token to be possible, we can support
that with extra flag passed into BPF_TOKEN_CREATE.

Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231213190842.3844987-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/token.c b/kernel/bpf/token.c
index 17212efcde60b52582207d6c69a209fca749a4f7..a86fccd57e2d0c5915d0f6d470dc13c895d1b7bb 100644 (file)
--- a/kernel/bpf/token.c
+++ b/kernel/bpf/token.c
@@ -152,6 +152,15 @@ int bpf_token_create(union bpf_attr *attr)
                goto out_path;
        }
 
+       mnt_opts = path.dentry->d_sb->s_fs_info;
+       if (mnt_opts->delegate_cmds == 0 &&
+           mnt_opts->delegate_maps == 0 &&
+           mnt_opts->delegate_progs == 0 &&
+           mnt_opts->delegate_attachs == 0) {
+               err = -ENOENT; /* no BPF token delegation is set up */
+               goto out_path;
+       }
+
        mode = S_IFREG | ((S_IRUSR | S_IWUSR) & ~current_umask());
        inode = bpf_get_inode(path.mnt->mnt_sb, NULL, mode);
        if (IS_ERR(inode)) {
@@ -181,7 +190,6 @@ int bpf_token_create(union bpf_attr *attr)
        /* remember bpffs owning userns for future ns_capable() checks */
        token->userns = get_user_ns(userns);
 
-       mnt_opts = path.dentry->d_sb->s_fs_info;
        token->allowed_cmds = mnt_opts->delegate_cmds;
        token->allowed_maps = mnt_opts->delegate_maps;
        token->allowed_progs = mnt_opts->delegate_progs;