linux-kernel - [PATCH bpf] bpf: verifier: check for packet data access based on target prog

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Date:   Tue, 18 Aug 2020 18:12:44 -0700
From:   Udip Pant <udippant@...com>
To:     Udip Pant <udippant@...com>, Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        "David S . Miller" <davem@...emloft.net>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: [PATCH bpf] bpf: verifier: check for packet data access based on target prog

While using dynamic program extension (of type BPF_PROG_TYPE_EXT), we
need to check the program type of the target program to grant the read /
write access to the packet data.

The BPF_PROG_TYPE_EXT type can be used to extend types such as XDP, SKB
and others. Since the BPF_PROG_TYPE_EXT program type on itself is just a
placeholder for those, we need this extended check for those target
programs to actually work while using this option.

Tested this with a freplace xdp program. Without this patch, the
verifier fails with error 'cannot write into packet'.

Signed-off-by: Udip Pant <udippant@...com>
---
 kernel/bpf/verifier.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ef938f17b944..4d7604430994 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2629,7 +2629,11 @@ static bool may_access_direct_pkt_data(struct bpf_verifier_env *env,
 				       const struct bpf_call_arg_meta *meta,
 				       enum bpf_access_type t)
 {
-	switch (env->prog->type) {
+	struct bpf_prog *prog = env->prog;
+	enum bpf_prog_type prog_type = prog->aux->linked_prog ?
+	      prog->aux->linked_prog->type : prog->type;
+
+	switch (prog_type) {
 	/* Program types only with direct read access go here! */
 	case BPF_PROG_TYPE_LWT_IN:
 	case BPF_PROG_TYPE_LWT_OUT:
-- 
2.24.1