[PATCH bpf v2 0/3] Fix reference state management for synchronous callbacks

[PATCH bpf v2 0/3] Fix reference state management for synchronous callbacks

[Kumar Kartikeya Dwivedi]

This is patch 1, 2 + their individual tests split into a separate series from
the RFC, so that these can be taken in, while we continue working towards a fix
for handling stack access inside the callback.

Changelog:
----------
v1 -> v2:
v1: https://lore.kernel.org/bpf/20220822131923.21476-1-memxor@gmail.com

  * Fix error for test_progs-no_alu32 due to distinct alloc_insn in errstr

RFC v1 -> v1:
RFC v1: https://lore.kernel.org/bpf/20220815051540.18791-1-memxor@gmail.com

  * Fix up commit log to add more explanation (Alexei)
  * Split reference state fix out into a separate series

Kumar Kartikeya Dwivedi (3):
  bpf: Move bpf_loop and bpf_for_each_map_elem under CAP_BPF
  bpf: Fix reference state management for synchronous callbacks
  selftests/bpf: Add tests for reference state fixes for callbacks

 include/linux/bpf_verifier.h                  |  11 ++
 kernel/bpf/helpers.c                          |   8 +-
 kernel/bpf/verifier.c                         |  42 +++++--
 .../selftests/bpf/prog_tests/cb_refs.c        |  48 ++++++++
 tools/testing/selftests/bpf/progs/cb_refs.c   | 116 ++++++++++++++++++
 5 files changed, 212 insertions(+), 13 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/cb_refs.c
 create mode 100644 tools/testing/selftests/bpf/progs/cb_refs.c

-- 
2.34.1

[PATCH bpf v2 1/3] bpf: Move bpf_loop and bpf_for_each_map_elem under CAP_BPF

[Kumar Kartikeya Dwivedi]

They would require func_info which needs prog BTF anyway. Loading BTF
and setting the prog btf_fd while loading the prog indirectly requires
CAP_BPF, so just to reduce confusion, move both these helpers taking
callback under bpf_capable() protection as well, since they cannot be
used without CAP_BPF.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 kernel/bpf/helpers.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1f961f9982d2..d0e80926bac5 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1633,10 +1633,6 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 		return &bpf_ringbuf_submit_dynptr_proto;
 	case BPF_FUNC_ringbuf_discard_dynptr:
 		return &bpf_ringbuf_discard_dynptr_proto;
-	case BPF_FUNC_for_each_map_elem:
-		return &bpf_for_each_map_elem_proto;
-	case BPF_FUNC_loop:
-		return &bpf_loop_proto;
 	case BPF_FUNC_strncmp:
 		return &bpf_strncmp_proto;
 	case BPF_FUNC_dynptr_from_mem:
@@ -1675,6 +1671,10 @@ bpf_base_func_proto(enum bpf_func_id func_id)
 		return &bpf_timer_cancel_proto;
 	case BPF_FUNC_kptr_xchg:
 		return &bpf_kptr_xchg_proto;
+	case BPF_FUNC_for_each_map_elem:
+		return &bpf_for_each_map_elem_proto;
+	case BPF_FUNC_loop:
+		return &bpf_loop_proto;
 	default:
 		break;
 	}
-- 
2.34.1

[PATCH bpf v2 2/3] bpf: Fix reference state management for synchronous callbacks

[Kumar Kartikeya Dwivedi]

Currently, verifier verifies callback functions (sync and async) as if
they will be executed once, (i.e. it explores execution state as if the
function was being called once). The next insn to explore is set to
start of subprog and the exit from nested frame is handled using
curframe > 0 and prepare_func_exit. In case of async callback it uses a
customized variant of push_stack simulating a kind of branch to set up
custom state and execution context for the async callback.

While this approach is simple and works when callback really will be
executed only once, it is unsafe for all of our current helpers which
are for_each style, i.e. they execute the callback multiple times.

A callback releasing acquired references of the caller may do so
multiple times, but currently verifier sees it as one call inside the
frame, which then returns to caller. Hence, it thinks it released some
reference that the cb e.g. got access through callback_ctx (register
filled inside cb from spilled typed register on stack).

Similarly, it may see that an acquire call is unpaired inside the
callback, so the caller will copy the reference state of callback and
then will have to release the register with new ref_obj_ids. But again,
the callback may execute multiple times, but the verifier will only
account for acquired references for a single symbolic execution of the
callback, which will cause leaks.

Note that for async callback case, things are different. While currently
we have bpf_timer_set_callback which only executes it once, even for
multiple executions it would be safe, as reference state is NULL and
check_reference_leak would force program to release state before
BPF_EXIT. The state is also unaffected by analysis for the caller frame.
Hence async callback is safe.

Since we want the reference state to be accessible, e.g. for pointers
loaded from stack through callback_ctx's PTR_TO_STACK, we still have to
copy caller's reference_state to callback's bpf_func_state, but we
enforce that whatever references it adds to that reference_state has
been released before it hits BPF_EXIT. This requires introducing a new
callback_ref member in the reference state to distinguish between caller
vs callee references. Hence, check_reference_leak now errors out if it
sees we are in callback_fn and we have not released callback_ref refs.
Since there can be multiple nested callbacks, like frame 0 -> cb1 -> cb2
etc. we need to also distinguish between whether this particular ref
belongs to this callback frame or parent, and only error for our own, so
we store state->frameno (which is always non-zero for callbacks).

In short, callbacks can read parent reference_state, but cannot mutate
it, to be able to use pointers acquired by the caller. They must only
undo their changes (by releasing their own acquired_refs before
BPF_EXIT) on top of caller reference_state before returning (at which
point the caller and callback state will match anyway, so no need to
copy it back to caller).

Fixes: 69c87ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf_verifier.h | 11 ++++++++++
 kernel/bpf/verifier.c        | 42 ++++++++++++++++++++++++++++--------
 2 files changed, 44 insertions(+), 9 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 2e3bad8640dc..1fdddbf3546b 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -212,6 +212,17 @@ struct bpf_reference_state {
 	 * is used purely to inform the user of a reference leak.
 	 */
 	int insn_idx;
+	/* There can be a case like:
+	 * main (frame 0)
+	 *  cb (frame 1)
+	 *   func (frame 3)
+	 *    cb (frame 4)
+	 * Hence for frame 4, if callback_ref just stored boolean, it would be
+	 * impossible to distinguish nested callback refs. Hence store the
+	 * frameno and compare that to callback_ref in check_reference_leak when
+	 * exiting a callback function.
+	 */
+	int callback_ref;
 };
 
 /* state of the program:
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 096fdac70165..3e885ba88b02 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1086,6 +1086,7 @@ static int acquire_reference_state(struct bpf_verifier_env *env, int insn_idx)
 	id = ++env->id_gen;
 	state->refs[new_ofs].id = id;
 	state->refs[new_ofs].insn_idx = insn_idx;
+	state->refs[new_ofs].callback_ref = state->in_callback_fn ? state->frameno : 0;
 
 	return id;
 }
@@ -1098,6 +1099,9 @@ static int release_reference_state(struct bpf_func_state *state, int ptr_id)
 	last_idx = state->acquired_refs - 1;
 	for (i = 0; i < state->acquired_refs; i++) {
 		if (state->refs[i].id == ptr_id) {
+			/* Cannot release caller references in callbacks */
+			if (state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
+				return -EINVAL;
 			if (last_idx && i != last_idx)
 				memcpy(&state->refs[i], &state->refs[last_idx],
 				       sizeof(*state->refs));
@@ -6938,10 +6942,17 @@ static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
 		caller->regs[BPF_REG_0] = *r0;
 	}
 
-	/* Transfer references to the caller */
-	err = copy_reference_state(caller, callee);
-	if (err)
-		return err;
+	/* callback_fn frame should have released its own additions to parent's
+	 * reference state at this point, or check_reference_leak would
+	 * complain, hence it must be the same as the caller. There is no need
+	 * to copy it back.
+	 */
+	if (!callee->in_callback_fn) {
+		/* Transfer references to the caller */
+		err = copy_reference_state(caller, callee);
+		if (err)
+			return err;
+	}
 
 	*insn_idx = callee->callsite + 1;
 	if (env->log.level & BPF_LOG_LEVEL) {
@@ -7065,13 +7076,20 @@ record_func_key(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
 static int check_reference_leak(struct bpf_verifier_env *env)
 {
 	struct bpf_func_state *state = cur_func(env);
+	bool refs_lingering = false;
 	int i;
 
+	if (state->frameno && !state->in_callback_fn)
+		return 0;
+
 	for (i = 0; i < state->acquired_refs; i++) {
+		if (state->in_callback_fn && state->refs[i].callback_ref != state->frameno)
+			continue;
 		verbose(env, "Unreleased reference id=%d alloc_insn=%d\n",
 			state->refs[i].id, state->refs[i].insn_idx);
+		refs_lingering = true;
 	}
-	return state->acquired_refs ? -EINVAL : 0;
+	return refs_lingering ? -EINVAL : 0;
 }
 
 static int check_bpf_snprintf_call(struct bpf_verifier_env *env,
@@ -12332,6 +12350,16 @@ static int do_check(struct bpf_verifier_env *env)
 					return -EINVAL;
 				}
 
+				/* We must do check_reference_leak here before
+				 * prepare_func_exit to handle the case when
+				 * state->curframe > 0, it may be a callback
+				 * function, for which reference_state must
+				 * match caller reference state when it exits.
+				 */
+				err = check_reference_leak(env);
+				if (err)
+					return err;
+
 				if (state->curframe) {
 					/* exit from nested function */
 					err = prepare_func_exit(env, &env->insn_idx);
@@ -12341,10 +12369,6 @@ static int do_check(struct bpf_verifier_env *env)
 					continue;
 				}
 
-				err = check_reference_leak(env);
-				if (err)
-					return err;
-
 				err = check_return_code(env);
 				if (err)
 					return err;
-- 
2.34.1

[PATCH bpf v2 3/3] selftests/bpf: Add tests for reference state fixes for callbacks

[Kumar Kartikeya Dwivedi]

These are regression tests to ensure we don't end up in invalid runtime
state for helpers that execute callbacks multiple times. It exercises
the fixes to verifier callback handling for reference state in previous
patches.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/prog_tests/cb_refs.c        |  48 ++++++++
 tools/testing/selftests/bpf/progs/cb_refs.c   | 116 ++++++++++++++++++
 2 files changed, 164 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/cb_refs.c
 create mode 100644 tools/testing/selftests/bpf/progs/cb_refs.c

diff --git a/tools/testing/selftests/bpf/prog_tests/cb_refs.c b/tools/testing/selftests/bpf/prog_tests/cb_refs.c
new file mode 100644
index 000000000000..3bff680de16c
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/cb_refs.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "bpf/libbpf.h"
+#include <test_progs.h>
+#include <network_helpers.h>
+
+#include "cb_refs.skel.h"
+
+static char log_buf[1024 * 1024];
+
+struct {
+	const char *prog_name;
+	const char *err_msg;
+} cb_refs_tests[] = {
+	{ "underflow_prog", "reference has not been acquired before" },
+	{ "leak_prog", "Unreleased reference" },
+	{ "nested_cb", "Unreleased reference id=4 alloc_insn=2" }, /* alloc_insn=2{4,5} */
+	{ "non_cb_transfer_ref", "Unreleased reference id=4 alloc_insn=1" }, /* alloc_insn=1{1,2} */
+};
+
+void test_cb_refs(void)
+{
+	LIBBPF_OPTS(bpf_object_open_opts, opts, .kernel_log_buf = log_buf,
+						.kernel_log_size = sizeof(log_buf),
+						.kernel_log_level = 1);
+	struct bpf_program *prog;
+	struct cb_refs *skel;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(cb_refs_tests); i++) {
+		LIBBPF_OPTS(bpf_test_run_opts, run_opts,
+			.data_in = &pkt_v4,
+			.data_size_in = sizeof(pkt_v4),
+			.repeat = 1,
+		);
+		skel = cb_refs__open_opts(&opts);
+		if (!ASSERT_OK_PTR(skel, "cb_refs__open_and_load"))
+			return;
+		prog = bpf_object__find_program_by_name(skel->obj, cb_refs_tests[i].prog_name);
+		bpf_program__set_autoload(prog, true);
+		if (!ASSERT_ERR(cb_refs__load(skel), "cb_refs__load"))
+			bpf_prog_test_run_opts(bpf_program__fd(prog), &run_opts);
+		if (!ASSERT_OK_PTR(strstr(log_buf, cb_refs_tests[i].err_msg), "expected error message")) {
+			fprintf(stderr, "Expected: %s\n", cb_refs_tests[i].err_msg);
+			fprintf(stderr, "Verifier: %s\n", log_buf);
+		}
+		cb_refs__destroy(skel);
+	}
+}
diff --git a/tools/testing/selftests/bpf/progs/cb_refs.c b/tools/testing/selftests/bpf/progs/cb_refs.c
new file mode 100644
index 000000000000..7653df1bc787
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/cb_refs.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+struct map_value {
+	struct prog_test_ref_kfunc __kptr_ref *ptr;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value);
+	__uint(max_entries, 16);
+} array_map SEC(".maps");
+
+extern struct prog_test_ref_kfunc *bpf_kfunc_call_test_acquire(unsigned long *sp) __ksym;
+extern void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p) __ksym;
+
+static __noinline int cb1(void *map, void *key, void *value, void *ctx)
+{
+	void *p = *(void **)ctx;
+	bpf_kfunc_call_test_release(p);
+	/* Without the fix this would cause underflow */
+	return 0;
+}
+
+SEC("?tc")
+int underflow_prog(void *ctx)
+{
+	struct prog_test_ref_kfunc *p;
+	unsigned long sl = 0;
+
+	p = bpf_kfunc_call_test_acquire(&sl);
+	if (!p)
+		return 0;
+	bpf_for_each_map_elem(&array_map, cb1, &p, 0);
+	return 0;
+}
+
+static __always_inline int cb2(void *map, void *key, void *value, void *ctx)
+{
+	unsigned long sl = 0;
+
+	*(void **)ctx = bpf_kfunc_call_test_acquire(&sl);
+	/* Without the fix this would leak memory */
+	return 0;
+}
+
+SEC("?tc")
+int leak_prog(void *ctx)
+{
+	struct prog_test_ref_kfunc *p;
+	struct map_value *v;
+	unsigned long sl;
+
+	v = bpf_map_lookup_elem(&array_map, &(int){0});
+	if (!v)
+		return 0;
+
+	p = NULL;
+	bpf_for_each_map_elem(&array_map, cb2, &p, 0);
+	p = bpf_kptr_xchg(&v->ptr, p);
+	if (p)
+		bpf_kfunc_call_test_release(p);
+	return 0;
+}
+
+static __always_inline int cb(void *map, void *key, void *value, void *ctx)
+{
+	return 0;
+}
+
+static __always_inline int cb3(void *map, void *key, void *value, void *ctx)
+{
+	unsigned long sl = 0;
+	void *p;
+
+	bpf_kfunc_call_test_acquire(&sl);
+	bpf_for_each_map_elem(&array_map, cb, &p, 0);
+	/* It should only complain here, not in cb. This is why we need
+	 * callback_ref to be set to frameno.
+	 */
+	return 0;
+}
+
+SEC("?tc")
+int nested_cb(void *ctx)
+{
+	struct prog_test_ref_kfunc *p;
+	unsigned long sl = 0;
+	int sp = 0;
+
+	p = bpf_kfunc_call_test_acquire(&sl);
+	if (!p)
+		return 0;
+	bpf_for_each_map_elem(&array_map, cb3, &sp, 0);
+	bpf_kfunc_call_test_release(p);
+	return 0;
+}
+
+SEC("?tc")
+int non_cb_transfer_ref(void *ctx)
+{
+	struct prog_test_ref_kfunc *p;
+	unsigned long sl = 0;
+
+	p = bpf_kfunc_call_test_acquire(&sl);
+	if (!p)
+		return 0;
+	cb1(NULL, NULL, NULL, &p);
+	bpf_kfunc_call_test_acquire(&sl);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.34.1

Re: [PATCH bpf v2 0/3] Fix reference state management for synchronous callbacks

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Tue, 23 Aug 2022 03:27:56 +0200 you wrote:
> This is patch 1, 2 + their individual tests split into a separate series from
> the RFC, so that these can be taken in, while we continue working towards a fix
> for handling stack access inside the callback.
> 
> Changelog:
> ----------
> v1 -> v2:
> v1: https://lore.kernel.org/bpf/20220822131923.21476-1-memxor@gmail.com
> 
> [...]

Here is the summary with links:
  - [bpf,v2,1/3] bpf: Move bpf_loop and bpf_for_each_map_elem under CAP_BPF
    https://git.kernel.org/bpf/bpf-next/c/5679ff2f138f
  - [bpf,v2,2/3] bpf: Fix reference state management for synchronous callbacks
    https://git.kernel.org/bpf/bpf-next/c/2e5e0e8ede02
  - [bpf,v2,3/3] selftests/bpf: Add tests for reference state fixes for callbacks
    https://git.kernel.org/bpf/bpf-next/c/3cf7e7d8685c

You are awesome, thank you!
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