[PATCH HID v2 15/16] HID: bpf: rework hid_bpf_ops_btf_struct_access

Benjamin Tissoires
2024-06-07T15:29:07Z

The idea is to provide a list of stucts and their editable fields.

Currently no functional changes are introduced here, we will add some
more writeable fields in the next patch.

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>

---

new in v2
---
drivers/hid/bpf/hid_bpf_struct_ops.c | 91 +++++++++++++++++++++++++++---------
1 file changed, 69 insertions(+), 22 deletions(-)

diff --git a/drivers/hid/bpf/hid_bpf_struct_ops.c b/drivers/hid/bpf/hid_bpf_struct_ops.c
index 056d05d96962..944e6d91a36b 100644
--- a/drivers/hid/bpf/hid_bpf_struct_ops.c
+++ b/drivers/hid/bpf/hid_bpf_struct_ops.c
@@ -16,6 +16,7 @@
#include <linux/hid_bpf.h>
#include <linux/init.h>
#include <linux/module.h>
+#include <linux/stddef.h>
#include <linux/workqueue.h>
#include "hid_bpf_dispatch.h"

@@ -52,40 +53,86 @@ static int hid_bpf_ops_check_member(const struct btf_type *t,
return 0;
}

+struct hid_bpf_offset_write_range {
+ const char *struct_name;
+ u32 struct_length;
+ u32 start;
+ u32 end;
+};
+
static int hid_bpf_ops_btf_struct_access(struct bpf_verifier_log *log,
const struct bpf_reg_state *reg,
int off, int size)
{
- const struct btf_type *state;
- const struct btf_type *t;
- s32 type_id;
+#define WRITE_RANGE(_name, _field, _start_offset, _end_offset) \
+ { \
+ .struct_name = #_name, \
+ .struct_length = sizeof(struct _name), \
+ .start = offsetof(struct _name, _field) + _start_offset, \
+ .end = offsetofend(struct _name, _field) + _end_offset, \
+ }

- type_id = btf_find_by_name_kind(reg->btf, "hid_bpf_ctx",
- BTF_KIND_STRUCT);
- if (type_id < 0)
- return -EINVAL;
+ const struct hid_bpf_offset_write_range write_ranges[] = {
+ WRITE_RANGE(hid_bpf_ctx, retval, 0, 0),
+ };
+#undef WRITE_RANGE
+ const struct btf_type *state = NULL;
+ const struct btf_type *t;
+ const char *cur = NULL;
+ int i;

t = btf_type_by_id(reg->btf, reg->btf_id);
- state = btf_type_by_id(reg->btf, type_id);
- if (t != state) {
- bpf_log(log, "only access to hid_bpf_ctx is supported\n");
- return -EACCES;
- }

- /* out-of-bound access in hid_bpf_ctx */
- if (off + size > sizeof(struct hid_bpf_ctx)) {
- bpf_log(log, "write access at off %d with size %d\n", off, size);
- return -EACCES;
+ for (i = 0; i < ARRAY_SIZE(write_ranges); i++) {
+ const struct hid_bpf_offset_write_range *write_range = &write_ranges[i];
+ s32 type_id;
+
+ /* we already found a writeable struct, but there is a
+ * new one, let's break the loop.
+ */
+ if (t == state && write_range->struct_name != cur)
+ break;
+
+ /* new struct to look for */
+ if (write_range->struct_name != cur) {
+ type_id = btf_find_by_name_kind(reg->btf, write_range->struct_name,
+ BTF_KIND_STRUCT);
+ if (type_id < 0)
+ return -EINVAL;
+
+ state = btf_type_by_id(reg->btf, type_id);
+ }
+
+ /* this is not the struct we are looking for */
+ if (t != state) {
+ cur = write_range->struct_name;
+ continue;
+ }
+
+ /* first time we see this struct, check for out of bounds */
+ if (cur != write_range->struct_name &&
+ off + size > write_range->struct_length) {
+ bpf_log(log, "write access for struct %s at off %d with size %d\n",
+ write_range->struct_name, off, size);
+ return -EACCES;
+ }
+
+ /* now check if we are in our boundaries */
+ if (off >= write_range->start && off + size <= write_range->end)
+ return NOT_INIT;
+
+ cur = write_range->struct_name;
}

- if (off < offsetof(struct hid_bpf_ctx, retval)) {
+
+ if (t != state)
+ bpf_log(log, "write access to this struct is not supported\n");
+ else
bpf_log(log,
- "write access at off %d with size %d on read-only part of hid_bpf_ctx\n",
- off, size);
- return -EACCES;
- }
+ "write access at off %d with size %d on read-only part of %s\n",
+ off, size, cur);

- return NOT_INIT;
+ return -EACCES;
}

static const struct bpf_verifier_ops hid_bpf_verifier_ops = {

--
2.44.0