[PATCH v2 bpf] selftests/bpf: Add tests to assert that netfilter progs cannot write to skb

[PATCH v2 bpf] selftests/bpf: Add tests to assert that netfilter progs cannot write to skb

[Florian Westphal]

The netfilter framework is allergic to ip header changing after
validation done by ip/ipv6 stack.

Assert that bpf netfilter programs do not allow skb write access.

Following additional tests are expected to be rejected by verifier:

1. alter skb->len.
2. alter skb->data.
3. prog calls bpf_dynptr_slice_rdwr.
4. alter location returned by dynptr API.

Add following test case for bpf runtime:
- alter skb data via bpf_dynptr_write()

Test checks via __retval() that bpf_dynptr_write() returned nonzero value.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 v2: follow LLM review: rename functions and clarify commit message.

 .../bpf/progs/verifier_netfilter_ctx.c        | 78 +++++++++++++++++++
 1 file changed, 78 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c b/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c
index e2cbc5bda65e..b5d7f567d0d4 100644
--- a/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c
+++ b/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c
@@ -113,4 +113,82 @@ int with_valid_ctx_access_test6(struct bpf_nf_ctx *ctx)
 	return th->dest == bpf_htons(22) ? NF_ACCEPT : NF_DROP;
 }
 
+SEC("netfilter")
+__description("netfilter test prog with skb write access")
+__failure __msg("only read is supported")
+int skb_len_write(struct bpf_nf_ctx *ctx)
+{
+	ctx->skb->len = 1;
+	return 1;
+}
+
+SEC("netfilter")
+__description("netfilter test prog with skb data write access")
+__failure __msg("cannot write into rdonly_untrusted_mem")
+int skb_data_write(struct bpf_nf_ctx *ctx)
+{
+	ctx->skb->data[0] = 0;
+	return 1;
+}
+
+SEC("netfilter")
+__description("netfilter test prog with bpf_dynptr_write")
+__success __failure_unpriv
+__retval(0)
+int with_dynptr_write(struct bpf_nf_ctx *ctx)
+{
+	struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;
+	struct bpf_dynptr ptr;
+	u8 buffer[1] = {};
+
+	if (bpf_dynptr_from_skb(skb, 0, &ptr))
+		return 1;
+
+	if (bpf_dynptr_write(&ptr, 0, buffer, sizeof(buffer), 0))
+		return 0; /* must always fail */
+
+	return 1;
+}
+
+SEC("netfilter")
+__description("netfilter test prog with bpf_dynptr_slice_rdwr")
+__failure __msg("the prog does not allow writes to packet data")
+int with_dynptr_rdwr(struct bpf_nf_ctx *ctx)
+{
+	struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;
+	u8 buffer_iph[20] = {};
+	struct bpf_dynptr ptr;
+	struct iphdr *iph;
+
+	if (bpf_dynptr_from_skb(skb, 0, &ptr))
+		return 1;
+
+	iph = bpf_dynptr_slice_rdwr(&ptr, 0, buffer_iph, sizeof(buffer_iph));
+	if (!iph)
+		return 0;
+
+	return 1;
+}
+
+SEC("netfilter")
+__description("netfilter test prog with bpf_dynptr_slice + write")
+__failure __msg("cannot write into rdonly_mem")
+int with_dynptr_store(struct bpf_nf_ctx *ctx)
+{
+	struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;
+	u8 buffer_iph[20] = {};
+	struct bpf_dynptr ptr;
+	struct iphdr *iph;
+
+	if (bpf_dynptr_from_skb(skb, 0, &ptr))
+		return 1;
+
+	iph = bpf_dynptr_slice(&ptr, 0, buffer_iph, sizeof(buffer_iph));
+	if (!iph)
+		return 0;
+	iph->protocol = 42;
+
+	return 1;
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.55.0

Re: [PATCH v2 bpf] selftests/bpf: Add tests to assert that netfilter progs cannot write to skb

[Jiayuan Chen]

on 9/3/26 2:58 PM, Florian Westphal wrote:
> The netfilter framework is allergic to ip header changing after
> validation done by ip/ipv6 stack.
>
> Assert that bpf netfilter programs do not allow skb write access.
>
> Following additional tests are expected to be rejected by verifier:
>
> 1. alter skb->len.
> 2. alter skb->data.
> 3. prog calls bpf_dynptr_slice_rdwr.
> 4. alter location returned by dynptr API.
>
> Add following test case for bpf runtime:
> - alter skb data via bpf_dynptr_write()
>
> Test checks via __retval() that bpf_dynptr_write() returned nonzero value.
>
> Signed-off-by: Florian Westphal <fw@strlen.de>


Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>


> ---
>   v2: follow LLM review: rename functions and clarify commit message.
>
>   .../bpf/progs/verifier_netfilter_ctx.c        | 78 +++++++++++++++++++
>   1 file changed, 78 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c b/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c
> index e2cbc5bda65e..b5d7f567d0d4 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c
> @@ -113,4 +113,82 @@ int with_valid_ctx_access_test6(struct bpf_nf_ctx *ctx)
>   	return th->dest == bpf_htons(22) ? NF_ACCEPT : NF_DROP;
>   }
>   
> +SEC("netfilter")
> +__description("netfilter test prog with skb write access")
> +__failure __msg("only read is supported")
> +int skb_len_write(struct bpf_nf_ctx *ctx)
> +{
> +	ctx->skb->len = 1;
> +	return 1;
> +}
> +
> +SEC("netfilter")
> +__description("netfilter test prog with skb data write access")
> +__failure __msg("cannot write into rdonly_untrusted_mem")
> +int skb_data_write(struct bpf_nf_ctx *ctx)
> +{
> +	ctx->skb->data[0] = 0;
> +	return 1;
> +}
> +
> +SEC("netfilter")
> +__description("netfilter test prog with bpf_dynptr_write")
> +__success __failure_unpriv
> +__retval(0)
> +int with_dynptr_write(struct bpf_nf_ctx *ctx)
> +{
> +	struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;
> +	struct bpf_dynptr ptr;
> +	u8 buffer[1] = {};
> +
> +	if (bpf_dynptr_from_skb(skb, 0, &ptr))
> +		return 1;
> +
> +	if (bpf_dynptr_write(&ptr, 0, buffer, sizeof(buffer), 0))
> +		return 0; /* must always fail */
> +
> +	return 1;
> +}
> +
> +SEC("netfilter")
> +__description("netfilter test prog with bpf_dynptr_slice_rdwr")
> +__failure __msg("the prog does not allow writes to packet data")
> +int with_dynptr_rdwr(struct bpf_nf_ctx *ctx)
> +{
> +	struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;
> +	u8 buffer_iph[20] = {};
> +	struct bpf_dynptr ptr;
> +	struct iphdr *iph;
> +
> +	if (bpf_dynptr_from_skb(skb, 0, &ptr))
> +		return 1;
> +
> +	iph = bpf_dynptr_slice_rdwr(&ptr, 0, buffer_iph, sizeof(buffer_iph));
> +	if (!iph)
> +		return 0;
> +
> +	return 1;
> +}
> +
> +SEC("netfilter")
> +__description("netfilter test prog with bpf_dynptr_slice + write")
> +__failure __msg("cannot write into rdonly_mem")
> +int with_dynptr_store(struct bpf_nf_ctx *ctx)
> +{
> +	struct __sk_buff *skb = (struct __sk_buff *)ctx->skb;
> +	u8 buffer_iph[20] = {};
> +	struct bpf_dynptr ptr;
> +	struct iphdr *iph;
> +
> +	if (bpf_dynptr_from_skb(skb, 0, &ptr))
> +		return 1;
> +
> +	iph = bpf_dynptr_slice(&ptr, 0, buffer_iph, sizeof(buffer_iph));
> +	if (!iph)
> +		return 0;
> +	iph->protocol = 42;
> +
> +	return 1;
> +}
> +
>   char _license[] SEC("license") = "GPL";

Re: [PATCH v2 bpf] selftests/bpf: Add tests to assert that netfilter progs cannot write to skb

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Thu,  3 Sep 2026 08:58:45 +0200 you wrote:
> The netfilter framework is allergic to ip header changing after
> validation done by ip/ipv6 stack.
> 
> Assert that bpf netfilter programs do not allow skb write access.
> 
> Following additional tests are expected to be rejected by verifier:
> 
> [...]

Here is the summary with links:
  - [v2,bpf] selftests/bpf: Add tests to assert that netfilter progs cannot write to skb
    https://git.kernel.org/bpf/bpf/c/254c881fe055

You are awesome, thank you!
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