Cover the store which used to be left as a plain BPF_STX without an
exception table entry:

  1: R1=trusted_ptr_Qdisc()
  ; struct Qdisc *next = sch->next_sched;
  1: (79) r1 = *(u64 *)(r1 +216)        ; R1=ptr_Qdisc()
  ; next->limit = 1000;
  3: (63) *(u32 *)(r1 +20) = r2         ; R1=ptr_Qdisc() R2=1000

Assert that it is rejected now.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t ns_bpf_qdisc
  [...]
  #257/1   ns_bpf_qdisc/fifo:OK
  #257/2   ns_bpf_qdisc/fq:OK
  #257/3   ns_bpf_qdisc/attach to mq:OK
  #257/4   ns_bpf_qdisc/attach to non root:OK
  #257/5   ns_bpf_qdisc/incompl_ops:OK
  #257/6   ns_bpf_qdisc/invalid_dynptr:OK
  #257/7   ns_bpf_qdisc/invalid_dynptr_cross_frame:OK
  #257/8   ns_bpf_qdisc/invalid_dynptr_slice:OK
  #257/9   ns_bpf_qdisc/untrusted_write:OK
  #257/10  ns_bpf_qdisc/dynptr_use_after_invalidate_clone:OK
  #257     ns_bpf_qdisc:OK
  Summary: 1/10 PASSED, 0 SKIPPED, 0/0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 v2->v3:
  - new patch

 .../selftests/bpf/prog_tests/bpf_qdisc.c      |  2 +
 .../progs/bpf_qdisc_fail__untrusted_write.c   | 65 +++++++++++++++++++
 2 files changed, 67 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_qdisc_fail__untrusted_write.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_qdisc.c b/tools/testing/selftests/bpf/prog_tests/bpf_qdisc.c
index 77f1c0550c9b..6dbd1487343c 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_qdisc.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_qdisc.c
@@ -11,6 +11,7 @@
 #include "bpf_qdisc_fail__invalid_dynptr.skel.h"
 #include "bpf_qdisc_fail__invalid_dynptr_slice.skel.h"
 #include "bpf_qdisc_fail__invalid_dynptr_cross_frame.skel.h"
+#include "bpf_qdisc_fail__untrusted_write.skel.h"
 #include "bpf_qdisc_dynptr_use_after_invalidate_clone.skel.h"
 
 #define LO_IFINDEX 1
@@ -230,6 +231,7 @@ void test_ns_bpf_qdisc(void)
 	RUN_TESTS(bpf_qdisc_fail__invalid_dynptr);
 	RUN_TESTS(bpf_qdisc_fail__invalid_dynptr_cross_frame);
 	RUN_TESTS(bpf_qdisc_fail__invalid_dynptr_slice);
+	RUN_TESTS(bpf_qdisc_fail__untrusted_write);
 	RUN_TESTS(bpf_qdisc_dynptr_use_after_invalidate_clone);
 }
 
diff --git a/tools/testing/selftests/bpf/progs/bpf_qdisc_fail__untrusted_write.c b/tools/testing/selftests/bpf/progs/bpf_qdisc_fail__untrusted_write.c
new file mode 100644
index 000000000000..688c2a049ae3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_qdisc_fail__untrusted_write.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <vmlinux.h>
+#include "bpf_experimental.h"
+#include "bpf_qdisc_common.h"
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("struct_ops")
+__failure __msg("only read is supported")
+int BPF_PROG(untrusted_write, struct sk_buff *skb, struct Qdisc *sch,
+	     struct bpf_sk_buff_ptr *to_free)
+{
+	struct Qdisc *next = sch->next_sched;
+
+	/*
+	 * sch is trusted, but the walk of next_sched yields a plain
+	 * PTR_TO_BTF_ID which may fault on a dereference. A store through
+	 * it does not get an exception table entry, there is no probed
+	 * store to rewrite it into, hence it has to be rejected before
+	 * bpf_qdisc_btf_struct_access() gets to allow the write to limit.
+	 */
+	next->limit = 1000;
+
+	bpf_qdisc_skb_drop(skb, to_free);
+	return NET_XMIT_DROP;
+}
+
+SEC("struct_ops")
+__auxiliary
+struct sk_buff *BPF_PROG(bpf_qdisc_test_dequeue, struct Qdisc *sch)
+{
+	return NULL;
+}
+
+SEC("struct_ops")
+__auxiliary
+int BPF_PROG(bpf_qdisc_test_init, struct Qdisc *sch, struct nlattr *opt,
+	     struct netlink_ext_ack *extack)
+{
+	return 0;
+}
+
+SEC("struct_ops")
+__auxiliary
+void BPF_PROG(bpf_qdisc_test_reset, struct Qdisc *sch)
+{
+}
+
+SEC("struct_ops")
+__auxiliary
+void BPF_PROG(bpf_qdisc_test_destroy, struct Qdisc *sch)
+{
+}
+
+SEC(".struct_ops")
+struct Qdisc_ops test = {
+	.enqueue   = (void *)untrusted_write,
+	.dequeue   = (void *)bpf_qdisc_test_dequeue,
+	.init      = (void *)bpf_qdisc_test_init,
+	.reset     = (void *)bpf_qdisc_test_reset,
+	.destroy   = (void *)bpf_qdisc_test_destroy,
+	.id        = "bpf_qdisc_test",
+};
-- 
2.43.0