linux-kernel - [PATCH 5.17 319/343] bpf: Treat bpf_sk_lookup remote

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Message-Id: <20220412063000.528180590@linuxfoundation.org>
Date:   Tue, 12 Apr 2022 08:32:17 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jakub Sitnicki <jakub@...udflare.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>
Subject: [PATCH 5.17 319/343] bpf: Treat bpf_sk_lookup remote_port as a 2-byte field

From: Jakub Sitnicki <jakub@...udflare.com>

commit 058ec4a7d9cf77238c73ad9f1e1a3ed9a29afcab upstream.

In commit 9a69e2b385f4 ("bpf: Make remote_port field in struct
bpf_sk_lookup 16-bit wide") the remote_port field has been split up and
re-declared from u32 to be16.

However, the accompanying changes to the context access converter have not
been well thought through when it comes big-endian platforms.

Today 2-byte wide loads from offsetof(struct bpf_sk_lookup, remote_port)
are handled as narrow loads from a 4-byte wide field.

This by itself is not enough to create a problem, but when we combine

 1. 32-bit wide access to ->remote_port backed by a 16-wide wide load, with
 2. inherent difference between litte- and big-endian in how narrow loads
    need have to be handled (see bpf_ctx_narrow_access_offset),

we get inconsistent results for a 2-byte loads from &ctx->remote_port on LE
and BE architectures. This in turn makes BPF C code for the common case of
2-byte load from ctx->remote_port not portable.

To rectify it, inform the context access converter that remote_port is
2-byte wide field, and only 1-byte loads need to be treated as narrow
loads.

At the same time, we special-case the 4-byte load from &ctx->remote_port to
continue handling it the same way as do today, in order to keep the
existing BPF programs working.

Fixes: 9a69e2b385f4 ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide")
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Martin KaFai Lau <kafai@...com>
Link: https://lore.kernel.org/bpf/20220319183356.233666-2-jakub@cloudflare.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/core/filter.c |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -10621,13 +10621,24 @@ static bool sk_lookup_is_valid_access(in
 	case bpf_ctx_range(struct bpf_sk_lookup, local_ip4):
 	case bpf_ctx_range_till(struct bpf_sk_lookup, remote_ip6[0], remote_ip6[3]):
 	case bpf_ctx_range_till(struct bpf_sk_lookup, local_ip6[0], local_ip6[3]):
-	case offsetof(struct bpf_sk_lookup, remote_port) ...
-	     offsetof(struct bpf_sk_lookup, local_ip4) - 1:
 	case bpf_ctx_range(struct bpf_sk_lookup, local_port):
 	case bpf_ctx_range(struct bpf_sk_lookup, ingress_ifindex):
 		bpf_ctx_record_field_size(info, sizeof(__u32));
 		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u32));
 
+	case bpf_ctx_range(struct bpf_sk_lookup, remote_port):
+		/* Allow 4-byte access to 2-byte field for backward compatibility */
+		if (size == sizeof(__u32))
+			return true;
+		bpf_ctx_record_field_size(info, sizeof(__be16));
+		return bpf_ctx_narrow_access_ok(off, size, sizeof(__be16));
+
+	case offsetofend(struct bpf_sk_lookup, remote_port) ...
+	     offsetof(struct bpf_sk_lookup, local_ip4) - 1:
+		/* Allow access to zero padding for backward compatibility */
+		bpf_ctx_record_field_size(info, sizeof(__u16));
+		return bpf_ctx_narrow_access_ok(off, size, sizeof(__u16));
+
 	default:
 		return false;
 	}
@@ -10709,6 +10720,11 @@ static u32 sk_lookup_convert_ctx_access(
 						     sport, 2, target_size));
 		break;
 
+	case offsetofend(struct bpf_sk_lookup, remote_port):
+		*target_size = 2;
+		*insn++ = BPF_MOV32_IMM(si->dst_reg, 0);
+		break;
+
 	case offsetof(struct bpf_sk_lookup, local_port):
 		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
 				      bpf_target_off(struct bpf_sk_lookup_kern,