[PATCH bpf v3 2/2] selftests/bpf: Add test for stale bounds on LSM retval context load

[Tristan Madani <tristmd@gmail.com>]

From: Tristan Madani <tristan@talencesecurity.com>

Add a verifier test that catches the stale-bounds issue fixed in the
previous patch. The test sets r6 = 0 to create known bounds, then loads
the LSM hook return value into r6 from the context. Without the fix,
the verifier intersects the retval range with the stale bounds and
incorrectly narrows r6 to a single value, pruning the fall-through
branch as dead code and missing the div-by-zero.

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 tools/testing/selftests/bpf/progs/verifier_lsm.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_lsm.c b/tools/testing/selftests/bpf/progs/verifier_lsm.c
index 2f8103bfa14e..c724bf389f5c 100644
--- a/tools/testing/selftests/bpf/progs/verifier_lsm.c
+++ b/tools/testing/selftests/bpf/progs/verifier_lsm.c
@@ -197,4 +197,19 @@ int BPF_PROG(sleepable_lsm_cgroup)
 	return 0;
 }
 
+SEC("lsm/file_mprotect")
+__description("lsm retval load must reset stale register bounds")
+__failure __msg("div by zero")
+__naked int retval_load_resets_bounds(void *ctx)
+{
+	asm volatile (
+	"r6 = 0;"
+	"r6 = *(u64 *)(r1 + 24);"
+	"if r6 == 0 goto +1;"
+	"r6 /= 0;"
+	"r0 = 0;"
+	"exit;"
+	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.47.3