bpf: Explicitly check accesses to bpf_sock_addr

Syzkaller found a kernel warning on the following sock_addr program:

    0: r0 = 0
    1: r2 = *(u32 *)(r1 +60)
    2: exit

which triggers:

    verifier bug: error during ctx access conversion (0)

This is happening because offset 60 in bpf_sock_addr corresponds to an
implicit padding of 4 bytes, right after msg_src_ip4. Access to this
padding isn't rejected in sock_addr_is_valid_access and it thus later
fails to convert the access.

This patch fixes it by explicitly checking the various fields of
bpf_sock_addr in sock_addr_is_valid_access.

I checked the other ctx structures and is_valid_access functions and
didn't find any other similar cases. Other cases of (properly handled)
padding are covered in new tests in a subsequent patch.

Fixes: 1cedee1 ("bpf: Hooks for sys_sendmsg")
Reported-by: syzbot+136ca59d411f92e821b7@syzkaller.appspotmail.com
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://syzkaller.appspot.com/bug?extid=136ca59d411f92e821b7
Link: https://lore.kernel.org/bpf/b58609d9490649e76e584b0361da0abd3c2c1779.1758094761.git.paul.chaignon@gmail.com

Author: pchaigno

net/core/filter.c

@@ -9284,13 +9284,17 @@ static bool sock_addr_is_valid_access(int off, int size,
 			return false;
 		info->reg_type = PTR_TO_SOCKET;
 		break;
-	default:
-		if (type == BPF_READ) {
-			if (size != size_default)
-				return false;
-		} else {
+	case bpf_ctx_range(struct bpf_sock_addr, user_family):
+	case bpf_ctx_range(struct bpf_sock_addr, family):
+	case bpf_ctx_range(struct bpf_sock_addr, type):
+	case bpf_ctx_range(struct bpf_sock_addr, protocol):
+		if (type != BPF_READ)
 			return false;
-		}
+		if (size != size_default)
+			return false;
+		break;
+	default:
+		return false;
 	}
 
 	return true;