bpf: Fix out of bounds access for ringbuf helpers

Both bpf_ringbuf_submit() and bpf_ringbuf_discard() have ARG_PTR_TO_ALLOC_MEM
in their bpf_func_proto definition as their first argument. They both expect
the result from a prior bpf_ringbuf_reserve() call which has a return type of
RET_PTR_TO_ALLOC_MEM_OR_NULL.

Meaning, after a NULL check in the code, the verifier will promote the register
type in the non-NULL branch to a PTR_TO_MEM and in the NULL branch to a known
zero scalar. Generally, pointer arithmetic on PTR_TO_MEM is allowed, so the
latter could have an offset.

The ARG_PTR_TO_ALLOC_MEM expects a PTR_TO_MEM register type. However, the non-
zero result from bpf_ringbuf_reserve() must be fed into either bpf_ringbuf_submit()
or bpf_ringbuf_discard() but with the original offset given it will then read
out the struct bpf_ringbuf_hdr mapping.

The verifier missed to enforce a zero offset, so that out of bounds access
can be triggered which could be used to escalate privileges if unprivileged
BPF was enabled (disabled by default in kernel).

Fixes: 457f4436 ("bpf: Implement BPF ring buffer and verifier support for it")
Reported-by: <tr3e.wang@gmail.com> (SecCoder Security Lab)
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -5318,9 +5318,15 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 	case PTR_TO_BUF:
 	case PTR_TO_BUF | MEM_RDONLY:
 	case PTR_TO_STACK:
+		/* Some of the argument types nevertheless require a
+		 * zero register offset.
+		 */
+		if (arg_type == ARG_PTR_TO_ALLOC_MEM)
+			goto force_off_check;
 		break;
 	/* All the rest must be rejected: */
 	default:
+force_off_check:
 		err = __check_ptr_off_reg(env, reg, regno,
 					  type == PTR_TO_BTF_ID);
 		if (err < 0)