As discussed in [0], it's dangerous to allow mapping BPF map, that's meant to be frozen and is read-only on BPF program side, because that allows user-space to actually store a writable view to the page even after it is frozen. This is exacerbated by BPF verifier making a strong assumption that contents of such frozen map will remain unchanged. To prevent this, disallow mapping BPF_F_RDONLY_PROG mmap()'able BPF maps as writable, ever. [0] https://lore.kernel.org/bpf/CAEf4BzYGWYhXdp6BJ7_=9OQPJxQpgug080MMjdSB72i9R+5c6g@mail.gmail.com/
Showing the changes to tools/testing (2 of 3 files). Show all
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| @@ -19,7 +19,7 @@ void test_mmap(void) · 18 unmodified lines | ||
| 19 | 19 | const size_t map_sz = roundup_page(sizeof(struct map_data)); |
| 20 | 20 | const int zero = 0, one = 1, two = 2, far = 1500; |
| 21 | 21 | const long page_size = sysconf(_SC_PAGE_SIZE); |
| 22 | − int err, duration = 0, i, data_map_fd, data_map_id, tmp_fd; | |
| 22 | + int err, duration = 0, i, data_map_fd, data_map_id, tmp_fd, rdmap_fd; | |
| 23 | 23 | struct bpf_map *data_map, *bss_map; |
| 24 | 24 | void *bss_mmaped = NULL, *map_mmaped = NULL, *tmp1, *tmp2; |
| 25 | 25 | struct test_mmap__bss *bss_data; |
| @@ -37,6 +37,17 @@ void test_mmap(void) · 11 unmodified lines | ||
| 37 | 37 | data_map = skel->maps.data_map; |
| 38 | 38 | data_map_fd = bpf_map__fd(data_map); |
| 39 | 39 | |
| 40 | + rdmap_fd = bpf_map__fd(skel->maps.rdonly_map); | |
| 41 | + tmp1 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, rdmap_fd, 0); | |
| 42 | + if (CHECK(tmp1 != MAP_FAILED, "rdonly_write_mmap", "unexpected success\n")) { | |
| 43 | + munmap(tmp1, 4096); | |
| 44 | + goto cleanup; | |
| 45 | + } | |
| 46 | + /* now double-check if it's mmap()'able at all */ | |
| 47 | + tmp1 = mmap(NULL, 4096, PROT_READ, MAP_SHARED, rdmap_fd, 0); | |
| 48 | + if (CHECK(tmp1 == MAP_FAILED, "rdonly_read_mmap", "failed: %d\n", errno)) | |
| 49 | + goto cleanup; | |
| 50 | + | |
| 40 | 51 | /* get map's ID */ |
| 41 | 52 | memset(&map_info, 0, map_info_sz); |
| 42 | 53 | err = bpf_obj_get_info_by_fd(data_map_fd, &map_info, &map_info_sz); |
| @@ -7,6 +7,14 @@ · 6 unmodified lines | ||
| 7 | 7 | |
| 8 | 8 | char _license[] SEC("license") = "GPL"; |
| 9 | 9 | |
| 10 | +struct { | |
| 11 | + __uint(type, BPF_MAP_TYPE_ARRAY); | |
| 12 | + __uint(max_entries, 4096); | |
| 13 | + __uint(map_flags, BPF_F_MMAPABLE | BPF_F_RDONLY_PROG); | |
| 14 | + __type(key, __u32); | |
| 15 | + __type(value, char); | |
| 16 | +} rdonly_map SEC(".maps"); | |
| 17 | + | |
| 10 | 18 | struct { |
| 11 | 19 | __uint(type, BPF_MAP_TYPE_ARRAY); |
| 12 | 20 | __uint(max_entries, 512 * 4); /* at least 4 pages of data */ |