[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 <redacted>
---
 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

On 8/18/20 6:12 PM, Udip Pant wrote:

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'.

Could you add a selftest for this? FYI, current selftests/bpf have
4 freplace programs:
   fexit_bpf2bpf.c:SEC("freplace/get_skb_len")
   fexit_bpf2bpf.c:SEC("freplace/get_skb_ifindex")
   fexit_bpf2bpf.c:SEC("freplace/get_constant")
   freplace_connect4.c:SEC("freplace/do_bind")