selftests/bpf: Add test for stale pkt range after scalar arithmetic

Extend the verifier_direct_packet_access BPF selftests to exercise the
verifier code paths which ensure that the pkt range is cleared after
add/sub alu with a known scalar. The tests reject the invalid access.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_direct
  [...]
  #592/35  verifier_direct_packet_access/direct packet access: pkt_range cleared after sub with known scalar:OK
  #592/36  verifier_direct_packet_access/direct packet access: pkt_range cleared after add with known scalar:OK
  #592/37  verifier_direct_packet_access/direct packet access: test3:OK
  #592/38  verifier_direct_packet_access/direct packet access: test3 @unpriv:OK
  #592/39  verifier_direct_packet_access/direct packet access: test34 (non-linear, cgroup_skb/ingress, too short eth):OK
  #592/40  verifier_direct_packet_access/direct packet access: test35 (non-linear, cgroup_skb/ingress, too short 1):OK
  #592/41  verifier_direct_packet_access/direct packet access: test36 (non-linear, cgroup_skb/ingress, long enough):OK
  #592     verifier_direct_packet_access:OK
  [...]
  Summary: 2/47 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260409155016.536608-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 61 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c b/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c
index 4ee3b7a708f7..915a9707298b 100644
--- a/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c
+++ b/tools/testing/selftests/bpf/progs/verifier_direct_packet_access.c
@@ -859,4 +859,65 @@ l0_%=:	r0 = 1;					\
 	: __clobber_all);
 }
 
+SEC("tc")
+__description("direct packet access: pkt_range cleared after sub with known scalar")
+__failure __msg("invalid access to packet")
+__naked void pkt_range_clear_after_sub(void)
+{
+	asm volatile ("					\
+	r9 = *(u32*)(r1 + %[__sk_buff_data]);		\
+	r8 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
+	r9 += 256;					\
+	if r9 >= r8 goto l0_%=;				\
+	r0 = 0;						\
+	exit;						\
+l0_%=:	/* r9 has AT_PKT_END (pkt + 256 >= pkt_end) */	\
+	r9 -= 256;					\
+	/*						\
+	 * AT_PKT_END must not survive the arithmetic.	\
+	 * is_pkt_ptr_branch_taken must validate both	\
+	 * branches when visiting the next condition.	\
+	 */						\
+	if r9 < r8 goto l1_%=;				\
+	r0 = 0;						\
+	exit;						\
+l1_%=:	r0 = *(u8*)(r9 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
+	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
+	: __clobber_all);
+}
+
+SEC("tc")
+__description("direct packet access: pkt_range cleared after add with known scalar")
+__failure __msg("invalid access to packet")
+__naked void pkt_range_clear_after_add(void)
+{
+	asm volatile ("					\
+	r9 = *(u32*)(r1 + %[__sk_buff_data]);		\
+	r8 = *(u32*)(r1 + %[__sk_buff_data_end]);	\
+	r9 += 256;					\
+	if r9 >= r8 goto l0_%=;				\
+	r0 = 0;						\
+	exit;						\
+l0_%=:	/* r9 has AT_PKT_END (pkt + 256 >= pkt_end) */	\
+	r9 += -256;					\
+	/*						\
+	 * Same as sub, but goes through BPF_ADD path.	\
+	 * AT_PKT_END must not survive the arithmetic.	\
+	 */						\
+	if r9 < r8 goto l1_%=;				\
+	r0 = 0;						\
+	exit;						\
+l1_%=:	r0 = *(u8*)(r9 + 0);				\
+	r0 = 0;						\
+	exit;						\
+"	:
+	: __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
+	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end))
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";