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bpf: Prevent mmap()'ing read-only maps as writable 

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/

Fixes: fc9702273e ("bpf: Add mmap() support for BPF_MAP_TYPE_ARRAY")
Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/bpf/20200519053824.1089415-1-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2020-05-18 22:38:24 -07:00 committed by Alexei Starovoitov
parent 0550cfe8c2
commit dfeb376dd4
3 changed files with 34 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
kernel/bpf/syscall.c
View File

@ -623,9 +623,20 @@ static int bpf_map_mmap(struct file *filp, struct vm_area_struct *vma)
mutex_lock(&map->freeze_mutex);
if ((vma->vm_flags & VM_WRITE) && map->frozen) {
err = -EPERM;
goto out;
if (vma->vm_flags & VM_WRITE) {
if (map->frozen) {
err = -EPERM;
goto out;
}
/* map is meant to be read-only, so do not allow mapping as
* writable, because it's possible to leak a writable page
* reference and allows user-space to still modify it after
* freezing, while verifier will assume contents do not change
*/
if (map->map_flags & BPF_F_RDONLY_PROG) {
err = -EACCES;
goto out;
}
}
/* set default open/close callbacks */

13
tools/testing/selftests/bpf/prog_tests/mmap.c
View File

@ -19,7 +19,7 @@ void test_mmap(void)
const size_t map_sz = roundup_page(sizeof(struct map_data));
const int zero = 0, one = 1, two = 2, far = 1500;
const long page_size = sysconf(_SC_PAGE_SIZE);
int err, duration = 0, i, data_map_fd, data_map_id, tmp_fd;
int err, duration = 0, i, data_map_fd, data_map_id, tmp_fd, rdmap_fd;
struct bpf_map *data_map, *bss_map;
void *bss_mmaped = NULL, *map_mmaped = NULL, *tmp1, *tmp2;
struct test_mmap__bss *bss_data;
@ -37,6 +37,17 @@ void test_mmap(void)
data_map = skel->maps.data_map;
data_map_fd = bpf_map__fd(data_map);
rdmap_fd = bpf_map__fd(skel->maps.rdonly_map);
tmp1 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, rdmap_fd, 0);
if (CHECK(tmp1 != MAP_FAILED, "rdonly_write_mmap", "unexpected success\n")) {
munmap(tmp1, 4096);
goto cleanup;
}
/* now double-check if it's mmap()'able at all */
tmp1 = mmap(NULL, 4096, PROT_READ, MAP_SHARED, rdmap_fd, 0);
if (CHECK(tmp1 == MAP_FAILED, "rdonly_read_mmap", "failed: %d\n", errno))
goto cleanup;
/* get map's ID */
memset(&map_info, 0, map_info_sz);
err = bpf_obj_get_info_by_fd(data_map_fd, &map_info, &map_info_sz);

8
tools/testing/selftests/bpf/progs/test_mmap.c
View File

@ -7,6 +7,14 @@
char _license[] SEC("license") = "GPL";
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 4096);
__uint(map_flags, BPF_F_MMAPABLE | BPF_F_RDONLY_PROG);
__type(key, __u32);
__type(value, char);
} rdonly_map SEC(".maps");
struct {
__uint(type, BPF_MAP_TYPE_ARRAY);
__uint(max_entries, 512 * 4); /* at least 4 pages of data */