selftests/bpf: add missing netfilter return value and ctx access tests

Extend prog_tests with two test cases:

 # ./test_progs --allow=verifier_netfilter_retcode
 #278/1   verifier_netfilter_retcode/bpf_exit with invalid return code. test1:OK
 #278/2   verifier_netfilter_retcode/bpf_exit with valid return code. test2:OK
 #278/3   verifier_netfilter_retcode/bpf_exit with valid return code. test3:OK
 #278/4   verifier_netfilter_retcode/bpf_exit with invalid return code. test4:OK
 #278     verifier_netfilter_retcode:OK

This checks that only accept and drop (0,1) are permitted.

NF_QUEUE could be implemented later if we can guarantee that attachment
of such programs can be rejected if they get attached to a pf/hook that
doesn't support async reinjection.

NF_STOLEN could be implemented via trusted helpers that can guarantee
that the skb will eventually be free'd.

v4: test case for bpf_nf_ctx access checks, requested by Alexei Starovoitov.
v5: also check ctx->{state,skb} can be dereferenced (Alexei).

 # ./test_progs --allow=verifier_netfilter_ctx
 #281/1   verifier_netfilter_ctx/netfilter invalid context access, size too short:OK
 #281/2   verifier_netfilter_ctx/netfilter invalid context access, size too short:OK
 #281/3   verifier_netfilter_ctx/netfilter invalid context access, past end of ctx:OK
 #281/4   verifier_netfilter_ctx/netfilter invalid context, write:OK
 #281/5   verifier_netfilter_ctx/netfilter valid context read and invalid write:OK
 #281/6   verifier_netfilter_ctx/netfilter test prog with skb and state read access:OK
 #281/7   verifier_netfilter_ctx/netfilter test prog with skb and state read access @unpriv:OK
 #281     verifier_netfilter_ctx:OK
Summary: 1/7 PASSED, 0 SKIPPED, 0 FAILED

This checks:
1/2: partial reads of ctx->{skb,state} are rejected
3. read access past sizeof(ctx) is rejected
4. write to ctx content, e.g. 'ctx->skb = NULL;' is rejected
5. ctx->state content cannot be altered
6. ctx->state and ctx->skb can be dereferenced
7. ... same program fails for unpriv (CAP_NET_ADMIN needed).

Link: https://lore.kernel.org/bpf/20230419021152.sjq4gttphzzy6b5f@dhcp-172-26-102-232.dhcp.thefacebook.com/
Link: https://lore.kernel.org/bpf/20230420201655.77kkgi3dh7fesoll@MacBook-Pro-6.local/Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20230421170300.24115-8-fw@strlen.deSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/verifier.c

@@ -29,6 +29,8 @@
 #include "verifier_map_ret_val.skel.h"
 #include "verifier_masking.skel.h"
 #include "verifier_meta_access.skel.h"
+#include "verifier_netfilter_ctx.skel.h"
+#include "verifier_netfilter_retcode.skel.h"
 #include "verifier_raw_stack.skel.h"
 #include "verifier_raw_tp_writable.skel.h"
 #include "verifier_reg_equal.skel.h"
@@ -103,6 +105,8 @@ void test_verifier_map_ptr(void)              { RUN(verifier_map_ptr); }
 void test_verifier_map_ret_val(void)          { RUN(verifier_map_ret_val); }
 void test_verifier_masking(void)              { RUN(verifier_masking); }
 void test_verifier_meta_access(void)          { RUN(verifier_meta_access); }
+void test_verifier_netfilter_ctx(void)        { RUN(verifier_netfilter_ctx); }
+void test_verifier_netfilter_retcode(void)    { RUN(verifier_netfilter_retcode); }
 void test_verifier_raw_stack(void)            { RUN(verifier_raw_stack); }
 void test_verifier_raw_tp_writable(void)      { RUN(verifier_raw_tp_writable); }
 void test_verifier_reg_equal(void)            { RUN(verifier_reg_equal); }

tools/testing/selftests/bpf/progs/verifier_netfilter_ctx.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+
+#include "bpf_misc.h"
+
+#include <bpf/bpf_endian.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("netfilter")
+__description("netfilter invalid context access, size too short")
+__failure __msg("invalid bpf_context access")
+__naked void with_invalid_ctx_access_test1(void)
+{
+	asm volatile ("					\
+	r2 = *(u8*)(r1 + %[__bpf_nf_ctx_state]);	\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__bpf_nf_ctx_state, offsetof(struct bpf_nf_ctx, state))
+	: __clobber_all);
+}
+
+SEC("netfilter")
+__description("netfilter invalid context access, size too short")
+__failure __msg("invalid bpf_context access")
+__naked void with_invalid_ctx_access_test2(void)
+{
+	asm volatile ("					\
+	r2 = *(u16*)(r1 + %[__bpf_nf_ctx_skb]);	\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__bpf_nf_ctx_skb, offsetof(struct bpf_nf_ctx, skb))
+	: __clobber_all);
+}
+
+SEC("netfilter")
+__description("netfilter invalid context access, past end of ctx")
+__failure __msg("invalid bpf_context access")
+__naked void with_invalid_ctx_access_test3(void)
+{
+	asm volatile ("					\
+	r2 = *(u64*)(r1 + %[__bpf_nf_ctx_size]);	\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__bpf_nf_ctx_size, sizeof(struct bpf_nf_ctx))
+	: __clobber_all);
+}
+
+SEC("netfilter")
+__description("netfilter invalid context, write")
+__failure __msg("invalid bpf_context access")
+__naked void with_invalid_ctx_access_test4(void)
+{
+	asm volatile ("					\
+	r2 = r1;					\
+	*(u64*)(r2 + 0) = r1;				\
+	r0 = 1;						\
+	exit;						\
+"	:
+	: __imm_const(__bpf_nf_ctx_skb, offsetof(struct bpf_nf_ctx, skb))
+	: __clobber_all);
+}
+
+#define NF_DROP 0
+#define NF_ACCEPT 1
+
+SEC("netfilter")
+__description("netfilter valid context read and invalid write")
+__failure __msg("only read is supported")
+int with_invalid_ctx_access_test5(struct bpf_nf_ctx *ctx)
+{
+	struct nf_hook_state *state = (void *)ctx->state;
+
+	state->sk = NULL;
+	return NF_ACCEPT;
+}
+
+extern int bpf_dynptr_from_skb(struct sk_buff *skb, __u64 flags,
+                               struct bpf_dynptr *ptr__uninit) __ksym;
+extern void *bpf_dynptr_slice(const struct bpf_dynptr *ptr, uint32_t offset,
+                                   void *buffer, uint32_t buffer__sz) __ksym;
+
+SEC("netfilter")
+__description("netfilter test prog with skb and state read access")
+__success __failure_unpriv
+__retval(0)
+int with_valid_ctx_access_test6(struct bpf_nf_ctx *ctx)
+{
+	const struct nf_hook_state *state = ctx->state;
+	struct sk_buff *skb = ctx->skb;
+	const struct iphdr *iph;
+	const struct tcphdr *th;
+	u8 buffer_iph[20] = {};
+	u8 buffer_th[40] = {};
+	struct bpf_dynptr ptr;
+	uint8_t ihl;
+
+	if (skb->len <= 20 || bpf_dynptr_from_skb(skb, 0, &ptr))
+		return NF_ACCEPT;
+
+	iph = bpf_dynptr_slice(&ptr, 0, buffer_iph, sizeof(buffer_iph));
+	if (!iph)
+		return NF_ACCEPT;
+
+	if (state->pf != 2)
+		return NF_ACCEPT;
+
+	ihl = iph->ihl << 2;
+
+	th = bpf_dynptr_slice(&ptr, ihl, buffer_th, sizeof(buffer_th));
+	if (!th)
+		return NF_ACCEPT;
+
+	return th->dest == bpf_htons(22) ? NF_ACCEPT : NF_DROP;
+}
+
+char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/verifier_netfilter_retcode.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+
+SEC("netfilter")
+__description("bpf_exit with invalid return code. test1")
+__failure __msg("R0 is not a known value")
+__naked void with_invalid_return_code_test1(void)
+{
+	asm volatile ("					\
+	r0 = *(u64*)(r1 + 0);				\
+	exit;						\
+"	::: __clobber_all);
+}
+
+SEC("netfilter")
+__description("bpf_exit with valid return code. test2")
+__success
+__naked void with_valid_return_code_test2(void)
+{
+	asm volatile ("					\
+	r0 = 0;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
+SEC("netfilter")
+__description("bpf_exit with valid return code. test3")
+__success
+__naked void with_valid_return_code_test3(void)
+{
+	asm volatile ("					\
+	r0 = 1;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
+SEC("netfilter")
+__description("bpf_exit with invalid return code. test4")
+__failure __msg("R0 has value (0x2; 0x0)")
+__naked void with_invalid_return_code_test4(void)
+{
+	asm volatile ("					\
+	r0 = 2;						\
+	exit;						\
+"	::: __clobber_all);
+}