[PATCH bpf v2] bpf: Reject sleepable kprobe_multi programs at attach time

[Varun R Mallya <varunrmallya@gmail.com>]

kprobe.multi programs run in atomic/RCU context and cannot sleep.
However, bpf_kprobe_multi_link_attach() did not validate whether the
program being attached had the sleepable flag set, allowing sleepable
helpers such as bpf_copy_from_user() to be invoked from a non-sleepable
context.

This causes a "sleeping function called from invalid context" splat:

  BUG: sleeping function called from invalid context at ./include/linux/uaccess.h:169
  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1787, name: sudo
  preempt_count: 1, expected: 0
  RCU nest depth: 2, expected: 0

Fix this by rejecting sleepable programs early in
bpf_kprobe_multi_link_attach(), before any further processing.

Also add a selftest that tries to load a sleepable kprobe_multi program
and it needs to be rejected for the test to pass.

Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
Signed-off-by: Varun R Mallya <varunrmallya@gmail.com>
---
 kernel/trace/bpf_trace.c                      |  4 ++
 .../bpf/prog_tests/kprobe_multi_test.c        | 41 +++++++++++++++++++
 .../bpf/progs/kprobe_multi_sleepable.c        | 13 ++++++
 3 files changed, 58 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/kprobe_multi_sleepable.c

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 0b040a417442..af7079aa0f36 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2752,6 +2752,10 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	if (!is_kprobe_multi(prog))
 		return -EINVAL;
 
+	/* kprobe_multi is not allowed to be sleepable. */
+	if (prog->sleepable)
+		return -EINVAL;
+
 	/* Writing to context is not allowed for kprobes. */
 	if (prog->aux->kprobe_write_ctx)
 		return -EINVAL;
diff --git a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
index 78c974d4ea33..f02fec2b6fda 100644
--- a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
+++ b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
@@ -10,6 +10,7 @@
 #include "kprobe_multi_session_cookie.skel.h"
 #include "kprobe_multi_verifier.skel.h"
 #include "kprobe_write_ctx.skel.h"
+#include "kprobe_multi_sleepable.skel.h"
 #include "bpf/libbpf_internal.h"
 #include "bpf/hashmap.h"
 
@@ -633,6 +634,44 @@ static void test_attach_write_ctx(void)
 }
 #endif
 
+static void test_attach_multi_sleepable(void)
+{
+	struct kprobe_multi_sleepable *skel;
+	int err;
+
+	skel = kprobe_multi_sleepable__open();
+	if (!ASSERT_OK_PTR(skel, "kprobe_multi_sleepable__open"))
+		return;
+
+	err = bpf_program__set_flags(skel->progs.handle_kprobe_multi_sleepable,
+				     BPF_F_SLEEPABLE);
+	if (!ASSERT_OK(err, "bpf_program__set_flags"))
+		goto cleanup;
+
+	/* Load should succeed even with BPF_F_SLEEPABLE for KPROBE types */
+	err = kprobe_multi_sleepable__load(skel);
+	if (!ASSERT_OK(err, "kprobe_multi_sleepable__load"))
+		goto cleanup;
+
+	/* Attachment must fail for kprobe.multi + BPF_F_SLEEPABLE.
+	 * Also chosen a stable symbol to send into opts
+	 */
+	LIBBPF_OPTS(bpf_kprobe_multi_opts, opts);
+	const char *sym = "vfs_read";
+
+	opts.syms = &sym;
+	opts.cnt = 1;
+
+	skel->links.handle_kprobe_multi_sleepable =
+		bpf_program__attach_kprobe_multi_opts(skel->progs.handle_kprobe_multi_sleepable,
+						      NULL, &opts);
+	ASSERT_ERR_PTR(skel->links.handle_kprobe_multi_sleepable,
+		       "bpf_program__attach_kprobe_multi_opts");
+
+cleanup:
+	kprobe_multi_sleepable__destroy(skel);
+}
+
 void serial_test_kprobe_multi_bench_attach(void)
 {
 	if (test__start_subtest("kernel"))
@@ -676,5 +715,7 @@ void test_kprobe_multi_test(void)
 		test_unique_match();
 	if (test__start_subtest("attach_write_ctx"))
 		test_attach_write_ctx();
+	if (test__start_subtest("attach_multi_sleepable"))
+		test_attach_multi_sleepable();
 	RUN_TESTS(kprobe_multi_verifier);
 }
diff --git a/tools/testing/selftests/bpf/progs/kprobe_multi_sleepable.c b/tools/testing/selftests/bpf/progs/kprobe_multi_sleepable.c
new file mode 100644
index 000000000000..56973ad8779d
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/kprobe_multi_sleepable.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+SEC("kprobe.multi")
+int handle_kprobe_multi_sleepable(struct pt_regs *ctx)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.53.0