Sashiko reported [1]:
This is a pre-existing issue, but does iterating over per-CPU maps expose
uninitialized kernel heap memory?
When working with per-CPU maps, temporary buffers are allocated using kmalloc
without the __GFP_ZERO flag in functions like bpf_iter_init_array_map in
kernel/bpf/arraymap.c:
kernel/bpf/arraymap.c:bpf_iter_init_array_map() {
...
value_buf = kmalloc(buf_size, GFP_USER | __GFP_NOWARN);
...
}
This is also done in kernel/bpf/hashtab.c:bpf_iter_init_hash_map().
If the map contains a BTF record, bpf_obj_memcpy in include/linux/bpf.h
explicitly stops at map->value_size instead of filling the entire rounded-up
size:
include/linux/bpf.h:bpf_obj_memcpy() {
...
memcpy(dst + curr_off, src + curr_off, size - curr_off);
...
}
This fails to overwrite the padding bytes up to round_up(map->value_size, 8).
[1] https://lore.kernel.org/bpf/[email protected]/
===
For example,
struct map_uninit_value {
struct prog_test_ref_kfunc __kptr_untrusted *unref_ptr;
__u32 data;
} __attribute__((packed));
struct {
__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
__type(key, int);
__type(value, struct map_uninit_value);
__uint(max_entries, 1);
} pcpu_array SEC(".maps");
There are 4 padding bytes in the kernel percpu_array map elements.
When lookup element from 'pcpu_array' map, for each CPU, the 4 padding
bytes memory allocated by syscall.c::map_lookup_elem():kvmalloc() could
be exposed to user space.
Without the fix, the selftest could fail with:
test_map_uninit_mem_exposure:FAIL:zeroed tail bytes unexpected memory
mismatch
actual:
2B 2B 2B 2B
expected:
00 00 00 00
Leon Hwang (2):
bpf: Copy per-CPU map value padding in copy_map_value_long()
selftests/bpf: Verify no non-zeroed kernel heap memory exposure
include/linux/bpf.h | 4 +-
.../bpf/prog_tests/test_map_uninit.c | 68 +++++++++++++++++++
tools/testing/selftests/bpf/progs/map_kptr.c | 12 ++++
3 files changed, 82 insertions(+), 2 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/test_map_uninit.c
--
2.54.0
