When accessing an array, even if you insert your own bounds check, sometimes the compiler will remove the check. bpf_cmp() will force the compiler to do the check. However, the compiler is free to make a copy of a register, check the copy, and use the original to access the array. The verifier knows the *copy* is within bounds, but not the original register! Although I couldn't recreate the "bounds check a copy of a register", the test below managed to get the compiler to spill a register to the stack, then bounds-check the register, and later reread the register - sans bounds check. By performing the bounds check and the indexing in assembly, we ensure the register used to index the array was bounds checked. Signed-off-by: Barret Rhoden <brho@xxxxxxxxxx> --- v2: https://lore.kernel.org/bpf/20240103185403.610641-1-brho@xxxxxxxxxx Changes since v2: - added a test prog that should load, but fails to verify for me (Debian clang version 16.0.6 (16)). these tests might be brittle and start successfully verifying for other compiler versions. - removed the mmap-an-arraymap patch - removed macros and added some "test fixture" code - used RUN_TESTS for the __failure cases .../bpf/prog_tests/test_array_elem.c | 167 ++++++++++++ .../selftests/bpf/progs/array_elem_test.c | 256 ++++++++++++++++++ tools/testing/selftests/bpf/progs/bpf_misc.h | 43 +++ 3 files changed, 466 insertions(+) create mode 100644 tools/testing/selftests/bpf/prog_tests/test_array_elem.c create mode 100644 tools/testing/selftests/bpf/progs/array_elem_test.c diff --git a/tools/testing/selftests/bpf/prog_tests/test_array_elem.c b/tools/testing/selftests/bpf/prog_tests/test_array_elem.c new file mode 100644 index 000000000000..93e8f03fdeac --- /dev/null +++ b/tools/testing/selftests/bpf/prog_tests/test_array_elem.c @@ -0,0 +1,167 @@ +// SPDX-License-Identifier: GPL-2.0 +/* Copyright (c) 2024 Google LLC. */ +#include <test_progs.h> +#include "array_elem_test.skel.h" + +#include <sys/mman.h> + +#define NR_MAP_ELEMS 100 + +static size_t map_mmap_sz(struct bpf_map *map) +{ + size_t mmap_sz; + + mmap_sz = (size_t)roundup(bpf_map__value_size(map), 8) * + bpf_map__max_entries(map); + mmap_sz = roundup(mmap_sz, sysconf(_SC_PAGE_SIZE)); + + return mmap_sz; +} + +static void *map_mmap(struct bpf_map *map) +{ + return mmap(NULL, map_mmap_sz(map), PROT_READ | PROT_WRITE, MAP_SHARED, + bpf_map__fd(map), 0); +} + +static void map_munmap(struct bpf_map *map, void *addr) +{ + munmap(addr, map_mmap_sz(map)); +} + +struct arr_elem_fixture { + struct array_elem_test *skel; + int *map_elems; +}; + +static void setup_fixture(struct arr_elem_fixture *tf, size_t prog_off) +{ + struct array_elem_test *skel; + struct bpf_program *prog; + int err; + + skel = array_elem_test__open(); + if (!ASSERT_OK_PTR(skel, "array_elem_test open")) + return; + + /* + * Our caller doesn't know the addr of the program until the skeleton is + * opened. But the offset to the pointer is statically known. + */ + prog = *(struct bpf_program**)((__u8*)skel + prog_off); + bpf_program__set_autoload(prog, true); + + err = array_elem_test__load(skel); + if (!ASSERT_EQ(err, 0, "array_elem_test load")) { + array_elem_test__destroy(skel); + return; + } + + err = array_elem_test__attach(skel); + if (!ASSERT_EQ(err, 0, "array_elem_test attach")) { + array_elem_test__destroy(skel); + return; + } + + for (int i = 0; i < NR_MAP_ELEMS; i++) { + skel->bss->lookup_indexes[i] = i; + err = bpf_map_update_elem(bpf_map__fd(skel->maps.lookup_again), + &i, &i, BPF_ANY); + ASSERT_EQ(err, 0, "array_elem_test set lookup_again"); + } + + tf->map_elems = map_mmap(skel->maps.arraymap); + ASSERT_OK_PTR(tf->map_elems, "mmap"); + + tf->skel = skel; +} + +static void run_test(struct arr_elem_fixture *tf) +{ + tf->skel->bss->target_pid = getpid(); + usleep(1); +} + +static void destroy_fixture(struct arr_elem_fixture *tf) +{ + map_munmap(tf->skel->maps.arraymap, tf->map_elems); + array_elem_test__destroy(tf->skel); +} + +static void test_access_single(void) +{ + struct arr_elem_fixture tf[1]; + + setup_fixture(tf, offsetof(struct array_elem_test, + progs.access_single)); + run_test(tf); + + ASSERT_EQ(tf->map_elems[0], 1337, "array_elem map value not written"); + + destroy_fixture(tf); +} + +static void test_access_all(void) +{ + struct arr_elem_fixture tf[1]; + + setup_fixture(tf, offsetof(struct array_elem_test, + progs.access_all)); + run_test(tf); + + for (int i = 0; i < NR_MAP_ELEMS; i++) + ASSERT_EQ(tf->map_elems[i], i, + "array_elem map value not written"); + + destroy_fixture(tf); +} + +static void test_oob_access(void) +{ + struct arr_elem_fixture tf[1]; + + setup_fixture(tf, offsetof(struct array_elem_test, + progs.oob_access)); + run_test(tf); + + for (int i = 0; i < NR_MAP_ELEMS; i++) + ASSERT_EQ(tf->map_elems[i], 0, + "array_elem map value was written"); + + destroy_fixture(tf); +} + +static void test_infer_size(void) +{ + struct arr_elem_fixture tf[1]; + + setup_fixture(tf, offsetof(struct array_elem_test, + progs.infer_size)); + run_test(tf); + + for (int i = 0; i < NR_MAP_ELEMS; i++) + ASSERT_EQ(tf->map_elems[i], i, + "array_elem map value not written"); + + destroy_fixture(tf); +} + +void test_test_array_elem(void) +{ + if (test__start_subtest("real_access_single")) + test_access_single(); + if (test__start_subtest("real_access_all")) + test_access_all(); + if (test__start_subtest("real_oob_access")) + test_oob_access(); + if (test__start_subtest("real_infer_size")) + test_infer_size(); + + /* + * RUN_TESTS() will load the *bad* tests, marked with + * __failure, and ensure they fail to load. It will also load the + * *good* tests, which we already tested, so you'll see some tests twice + * in the output. + */ + RUN_TESTS(array_elem_test); +} diff --git a/tools/testing/selftests/bpf/progs/array_elem_test.c b/tools/testing/selftests/bpf/progs/array_elem_test.c new file mode 100644 index 000000000000..9cd90a3623e5 --- /dev/null +++ b/tools/testing/selftests/bpf/progs/array_elem_test.c @@ -0,0 +1,256 @@ +// SPDX-License-Identifier: GPL-2.0 +/* Copyright (c) 2024 Google LLC. */ + +#include <vmlinux.h> +#include <stdbool.h> +#include <bpf/bpf_helpers.h> +#include <bpf/bpf_tracing.h> +#include "bpf_misc.h" +#include "bpf_experimental.h" + +char _license[] SEC("license") = "GPL"; + +int target_pid = 0; + +#define NR_MAP_ELEMS 100 + +/* + * We want to test valid accesses into an array, but we also need to fool the + * verifier. If we just do for (i = 0; i < 100; i++), the verifier knows the + * value of i and can tell we're inside the array. + * + * This "lookup" array is just the values 0, 1, 2..., such that + * lookup_indexes[i] == i. (set by userspace). But the verifier doesn't know + * that. + */ +unsigned int lookup_indexes[NR_MAP_ELEMS]; + +/* + * This second lookup array also has the values 0, 1, 2. The extra layer of + * lookups seems to make the compiler work a little harder, and more likely to + * spill to the stack. + */ +struct { + __uint(type, BPF_MAP_TYPE_ARRAY); + __uint(max_entries, NR_MAP_ELEMS); + __type(key, u32); + __type(value, u32); + __uint(map_flags, BPF_F_MMAPABLE); +} lookup_again SEC(".maps"); + +struct map_array { + int elems[NR_MAP_ELEMS]; +}; + +/* + * This is an ARRAY_MAP of a single struct, and that struct is an array of + * elements. Userspace can mmap the map as if it was just a basic array of + * elements. Though if you make an ARRAY_MAP where the *values* are ints, don't + * forget that bpf map elements are rounded up to 8 bytes. + * + * Once you get the pointer to the base of the inner array, you can access all + * of the elements without another bpf_map_lookup_elem(), which is useful if you + * are operating on multiple elements while holding a spinlock. + */ +struct { + __uint(type, BPF_MAP_TYPE_ARRAY); + __uint(max_entries, 1); + __type(key, u32); + __type(value, struct map_array); + __uint(map_flags, BPF_F_MMAPABLE); +} arraymap SEC(".maps"); + +static struct map_array *get_map_array(void) +{ + int zero = 0; + + return bpf_map_lookup_elem(&arraymap, &zero); +} + +static int *get_map_elems(void) +{ + struct map_array *arr = get_map_array(); + + if (!arr) + return NULL; + return arr->elems; +} + +/* + * This is convoluted enough that the compiler may spill a register (r1) before + * bounds checking it. + */ +static void bad_set_elem(unsigned int which, int val) +{ + u32 idx_1; + u32 *idx_2p; + int *map_elems; + + if (which >= NR_MAP_ELEMS) + return; + + idx_1 = lookup_indexes[which]; + idx_2p = bpf_map_lookup_elem(&lookup_again, &idx_1); + if (!idx_2p) + return; + + /* + * reuse idx_1, which is often r1. if you use a new variable, e.g. + * idx_3 = *idx_2p, the compiler will pick a non-caller save register + * (e.g. r6), and won't spill it to the stack. + */ + idx_1 = *idx_2p; + + /* + * Whether we use bpf_cmp or a normal comparison, r1 might get spilled + * to the stack, *then* checked against NR_MAP_ELEMS. The verifier will + * know r1's bounds, but since the check happened after the spill, it + * doesn't know about the stack variable's bounds. + */ + if (bpf_cmp_unlikely(idx_1, >=, NR_MAP_ELEMS)) + return; + + /* + * This does a bpf_map_lookup_elem(), which is a function call, which + * necessitates spilling r1. + */ + map_elems = get_map_elems(); + if (map_elems) + map_elems[idx_1] = val; +} + +SEC("?tp/syscalls/sys_enter_nanosleep") +__failure +__msg("R0 unbounded memory access, make sure to bounds check any such access") +int bad_access_single(void *ctx) +{ + bad_set_elem(0, 1337); + return 0; +} + +SEC("?tp/syscalls/sys_enter_nanosleep") +__failure +__msg("R0 unbounded memory access, make sure to bounds check any such access") +int bad_access_all(void *ctx) +{ + for (int i = 0; i < NR_MAP_ELEMS; i++) + bad_set_elem(i, i); + return 0; +} + +/* + * Both lookup_indexes and lookup_again are identity maps, i.e. f(x) = x (within + * bounds), so ultimately we're setting map_elems[which] = val. + */ +static void good_set_elem(unsigned int which, int val) +{ + u32 idx_1; + u32 *idx_2p; + int *map_elems, *x; + + if (which >= NR_MAP_ELEMS) + return; + idx_1 = lookup_indexes[which]; + idx_2p = bpf_map_lookup_elem(&lookup_again, &idx_1); + + if (!idx_2p) + return; + + idx_1 = *idx_2p; + + map_elems = get_map_elems(); + x = bpf_array_elem(map_elems, NR_MAP_ELEMS, idx_1); + if (x) + *x = val; +} + +/* + * Test accessing a single element in the array with a convoluted lookup. + */ +SEC("?tp/syscalls/sys_enter_nanosleep") +int access_single(void *ctx) +{ + if ((bpf_get_current_pid_tgid() >> 32) != target_pid) + return 0; + + good_set_elem(0, 1337); + + return 0; +} + +/* + * Test that we can access all elements, and that we are accessing the element + * we think we are accessing. + */ +SEC("?tp/syscalls/sys_enter_nanosleep") +int access_all(void *ctx) +{ + if ((bpf_get_current_pid_tgid() >> 32) != target_pid) + return 0; + + for (int i = 0; i < NR_MAP_ELEMS; i++) + good_set_elem(i, i); + + return 0; +} + +/* + * Helper for various OOB tests. An out-of-bound access should be handled like + * a lookup failure. Specifically, the verifier should ensure we do not access + * outside the array. Userspace will check that we didn't access somewhere + * inside the array. + */ +static void set_elem_to_1(long idx) +{ + int *map_elems = get_map_elems(); + int *x; + + x = bpf_array_elem(map_elems, NR_MAP_ELEMS, idx); + if (x) + *x = 1; +} + +/* + * Test various out-of-bounds accesses. + */ +SEC("?tp/syscalls/sys_enter_nanosleep") +int oob_access(void *ctx) +{ + if ((bpf_get_current_pid_tgid() >> 32) != target_pid) + return 0; + + set_elem_to_1(NR_MAP_ELEMS + 5); + set_elem_to_1(NR_MAP_ELEMS); + set_elem_to_1(-1); + set_elem_to_1(~0UL); + + return 0; +} + +/* + * Test that we can use the ARRAY_SIZE-style helper with an array in a map. + * + * Note that you cannot infer the size of the array from just a pointer; you + * have to use the actual elems[100]. i.e. this will fail and should fail to + * compile (-Wsizeof-pointer-div): + * + * int *map_elems = get_map_elems(); + * x = bpf_array_sz_elem(map_elems, lookup_indexes[i]); + */ +SEC("?tp/syscalls/sys_enter_nanosleep") +int infer_size(void *ctx) +{ + struct map_array *arr = get_map_array(); + int *x; + + if ((bpf_get_current_pid_tgid() >> 32) != target_pid) + return 0; + + for (int i = 0; i < NR_MAP_ELEMS; i++) { + x = bpf_array_sz_elem(arr->elems, lookup_indexes[i]); + if (x) + *x = i; + } + + return 0; +} diff --git a/tools/testing/selftests/bpf/progs/bpf_misc.h b/tools/testing/selftests/bpf/progs/bpf_misc.h index 2fd59970c43a..002bab44cde2 100644 --- a/tools/testing/selftests/bpf/progs/bpf_misc.h +++ b/tools/testing/selftests/bpf/progs/bpf_misc.h @@ -135,4 +135,47 @@ /* make it look to compiler like value is read and written */ #define __sink(expr) asm volatile("" : "+g"(expr)) +/* + * Access an array element within a bound, such that the verifier knows the + * access is safe. + * + * This macro asm is the equivalent of: + * + * if (!arr) + * return NULL; + * if (idx >= arr_sz) + * return NULL; + * return &arr[idx]; + * + * The index (___idx below) needs to be a u64, at least for certain versions of + * the BPF ISA, since there aren't u32 conditional jumps. + */ +#define bpf_array_elem(arr, arr_sz, idx) ({ \ + typeof(&(arr)[0]) ___arr = arr; \ + __u64 ___idx = idx; \ + if (___arr) { \ + asm volatile("if %[__idx] >= %[__bound] goto 1f; \ + %[__idx] *= %[__size]; \ + %[__arr] += %[__idx]; \ + goto 2f; \ + 1:; \ + %[__arr] = 0; \ + 2: \ + " \ + : [__arr]"+r"(___arr), [__idx]"+r"(___idx) \ + : [__bound]"r"((arr_sz)), \ + [__size]"i"(sizeof(typeof((arr)[0]))) \ + : "cc"); \ + } \ + ___arr; \ +}) + +/* + * Convenience wrapper for bpf_array_elem(), where we compute the size of the + * array. Be sure to use an actual array, and not a pointer, just like with the + * ARRAY_SIZE macro. + */ +#define bpf_array_sz_elem(arr, idx) \ + bpf_array_elem(arr, sizeof(arr) / sizeof((arr)[0]), idx) + #endif -- 2.43.0.472.g3155946c3a-goog
