2020-08-19 01:16:15

by Udip Pant

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 <[email protected]>
---
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

2020-08-19 06:05:01

by Yonghong Song

Subject: Re: [PATCH bpf] bpf: verifier: check for packet data access based on target prog

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")


>
> Signed-off-by: Udip Pant <[email protected]>
> ---
> 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:
>