FAILED: patch "[PATCH] bpf: Fix out of bounds access for ringbuf helpers" failed to apply to 5.16-stable tree

The patch below does not apply to the 5.16-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 64620e0a1e712a778095bd35cbb277dc2259281f Mon Sep 17 00:00:00 2001
From: Daniel Borkmann <[email protected]>
Date: Tue, 11 Jan 2022 14:43:41 +0000
Subject: [PATCH] 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).

index e0b3f4d683eb..c72c57a6684f 100644
@@ -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)