Merge branch 'bpf: fix NULL dereference during extable search'

Krister Johansen says:

====================
Hi,
Enclosed are a pair of patches for an oops that can occur if an exception is
generated while a bpf subprogram is running.  One of the bpf_prog_aux entries
for the subprograms are missing an extable.  This can lead to an exception that
would otherwise be handled turning into a NULL pointer bug.

These changes were tested via the verifier and progs selftests and no
regressions were observed.

Changes from v4:
- Ensure that num_exentries is copied to prog->aux from func[0] (Feedback from
  Ilya Leoshkevich)

Changes from v3:
- Selftest style fixups (Feedback from Yonghong Song)
- Selftest needs to assert that test bpf program executed (Feedback from
  Yonghong Song)
- Selftest should combine open and load using open_and_load (Feedback from
  Yonghong Song)

Changes from v2:
- Insert only the main program's kallsyms (Feedback from Yonghong Song and
  Alexei Starovoitov)
- Selftest should use ASSERT instead of CHECK (Feedback from Yonghong Song)
- Selftest needs some cleanup (Feedback from Yonghong Song)
- Switch patch order (Feedback from Alexei Starovoitov)

Changes from v1:
- Add a selftest (Feedback From Alexei Starovoitov)
- Move to a 1-line verifier change instead of searching multiple extables
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -17217,9 +17217,10 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 	}
 
 	/* finally lock prog and jit images for all functions and
-	 * populate kallsysm
+	 * populate kallsysm. Begin at the first subprogram, since
+	 * bpf_prog_load will add the kallsyms for the main program.
 	 */
-	for (i = 0; i < env->subprog_cnt; i++) {
+	for (i = 1; i < env->subprog_cnt; i++) {
 		bpf_prog_lock_ro(func[i]);
 		bpf_prog_kallsyms_add(func[i]);
 	}
@@ -17245,6 +17246,8 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 	prog->jited = 1;
 	prog->bpf_func = func[0]->bpf_func;
 	prog->jited_len = func[0]->jited_len;
+	prog->aux->extable = func[0]->aux->extable;
+	prog->aux->num_exentries = func[0]->aux->num_exentries;
 	prog->aux->func = func;
 	prog->aux->func_cnt = env->subprog_cnt;
 	bpf_prog_jit_attempt_done(prog);

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

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "test_subprogs_extable.skel.h"
+
+void test_subprogs_extable(void)
+{
+	const int read_sz = 456;
+	struct test_subprogs_extable *skel;
+	int err;
+
+	skel = test_subprogs_extable__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_and_load"))
+		return;
+
+	err = test_subprogs_extable__attach(skel);
+	if (!ASSERT_OK(err, "skel_attach"))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	ASSERT_OK(trigger_module_test_read(read_sz), "trigger_read");
+
+	ASSERT_NEQ(skel->bss->triggered, 0, "verify at least one program ran");
+
+	test_subprogs_extable__detach(skel);
+
+cleanup:
+	test_subprogs_extable__destroy(skel);
+}

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

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 8);
+	__type(key, __u32);
+	__type(value, __u64);
+} test_array SEC(".maps");
+
+unsigned int triggered;
+
+static __u64 test_cb(struct bpf_map *map, __u32 *key, __u64 *val, void *data)
+{
+	return 1;
+}
+
+SEC("fexit/bpf_testmod_return_ptr")
+int BPF_PROG(handle_fexit_ret_subprogs, int arg, struct file *ret)
+{
+	*(volatile long *)ret;
+	*(volatile int *)&ret->f_mode;
+	bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
+	triggered++;
+	return 0;
+}
+
+SEC("fexit/bpf_testmod_return_ptr")
+int BPF_PROG(handle_fexit_ret_subprogs2, int arg, struct file *ret)
+{
+	*(volatile long *)ret;
+	*(volatile int *)&ret->f_mode;
+	bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
+	triggered++;
+	return 0;
+}
+
+SEC("fexit/bpf_testmod_return_ptr")
+int BPF_PROG(handle_fexit_ret_subprogs3, int arg, struct file *ret)
+{
+	*(volatile long *)ret;
+	*(volatile int *)&ret->f_mode;
+	bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
+	triggered++;
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";