From 5679ff2f138f77b281c468959dc5022cc524d400 Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Tue, 23 Aug 2022 03:31:17 +0200 Subject: bpf: Move bpf_loop and bpf_for_each_map_elem under CAP_BPF 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 Link: https://lore.kernel.org/r/20220823013117.24916-1-memxor@gmail.com Signed-off-by: Alexei Starovoitov --- 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 2f4709378740..fc08035f14ed 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1613,10 +1613,6 @@ 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_strtol: @@ -1659,6 +1655,10 @@ 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; } -- cgit v1.2.3 From 9d9d00ac29d0ef7ce426964de46fa6b380357d0a Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Tue, 23 Aug 2022 03:31:25 +0200 Subject: bpf: Fix reference state management for synchronous callbacks 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: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper") Signed-off-by: Kumar Kartikeya Dwivedi Link: https://lore.kernel.org/r/20220823013125.24938-1-memxor@gmail.com Signed-off-by: Alexei Starovoitov --- 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 2c1f8069f7b7..0194a36d0b36 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1092,6 +1092,7 @@ 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; } @@ -1104,6 +1105,9 @@ 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)); @@ -6915,10 +6919,17 @@ 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) { @@ -7042,13 +7053,20 @@ 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, @@ -12337,6 +12355,16 @@ 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); @@ -12346,10 +12374,6 @@ 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; -- cgit v1.2.3 From 35f14dbd2fc6619dea8ac9eea18976378b18450b Mon Sep 17 00:00:00 2001 From: Kumar Kartikeya Dwivedi Date: Tue, 23 Aug 2022 03:32:26 +0200 Subject: selftests/bpf: Add tests for reference state fixes for callbacks 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 Link: https://lore.kernel.org/r/20220823013226.24988-1-memxor@gmail.com Signed-off-by: Alexei Starovoitov --- tools/testing/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 +#include + +#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 +#include +#include + +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"; -- cgit v1.2.3