netdev - [PATCH bpf-next] bpf: allow readonly direct path access for skfilter

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Message-Id: <20211123205607.452497-1-zenczykowski@gmail.com>
Date:   Tue, 23 Nov 2021 12:56:07 -0800
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Maciej Żenczykowski <maze@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Linux Network Development Mailing List <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        BPF Mailing List <bpf@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: [PATCH bpf-next] bpf: allow readonly direct path access for skfilter

From: Maciej Żenczykowski <maze@...gle.com>

skfilter bpf programs can read the packet directly via llvm.bpf.load.byte/
/half/word which are 8/16/32-bit primitive bpf instructions and thus
behave basically as well as DPA reads.  But there is no 64-bit equivalent,
due to the support for the equivalent 64-bit bpf opcode never having been
added (unclear why, there was a patch posted).
DPA uses a slightly different mechanism, so doesn't suffer this limitation.

Using 64-bit reads, 128-bit ipv6 address comparisons can be done in just
2 steps, instead of the 4 steps needed with llvm.bpf.word.

This should hopefully allow simpler (less instructions, and possibly less
logic and maybe even less jumps) programs.  Less jumps may also mean vastly
faster bpf verifier times (it can be exponential in the number of jumps...).

This can be particularly important when trying to do something like scan
a netlink message for a pattern (2000 iteration loop) to decide whether
a message should be dropped, or delivered to userspace (thus waking it up).

I'm requiring CAP_NET_ADMIN because I'm not sure of the security
implications...

Tested: only build tested
Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 331b170d9fcc..0c2e25fb9844 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3258,6 +3258,11 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
 	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);

 	switch (prog_type) {
+	case BPF_PROG_TYPE_SOCKET_FILTER:
+		if (meta || !capable(CAP_NET_ADMIN))
+			return false;
+		fallthrough;
+
 	/* Program types only with direct read access go here! */
 	case BPF_PROG_TYPE_LWT_IN:
 	case BPF_PROG_TYPE_LWT_OUT:
-- 
2.34.0.rc2.393.gf8c9666880-goog