bpf: Check the remaining info_cnt before repeating btf fields

When trying to repeat the btf fields for array of nested struct, it
doesn't check the remaining info_cnt. The following splat will be
reported when the value of ret * nelems is greater than BTF_FIELDS_MAX:

------------[ cut here ]------------
UBSAN: array-index-out-of-bounds in ../kernel/bpf/btf.c:3951:49
index 11 is out of range for type 'btf_field_info [11]'
CPU: 6 UID: 0 PID: 411 Comm: test_progs ...... 6.11.0-rc4+ #1
Tainted: [O]=OOT_MODULE
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ...
Call Trace:
<TASK>
dump_stack_lvl+0x57/0x70
dump_stack+0x10/0x20
ubsan_epilogue+0x9/0x40
__ubsan_handle_out_of_bounds+0x6f/0x80
? kallsyms_lookup_name+0x48/0xb0
btf_parse_fields+0x992/0xce0
map_create+0x591/0x770
__sys_bpf+0x229/0x2410
__x64_sys_bpf+0x1f/0x30
x64_sys_call+0x199/0x9f0
do_syscall_64+0x3b/0xc0
entry_SYSCALL_64_after_hwframe+0x4b/0x53
RIP: 0033:0x7fea56f2cc5d
......
</TASK>
---[ end trace ]---

Fix it by checking the remaining info_cnt in btf_repeat_fields() before
repeating the btf fields.

Fixes: 64e8ee814819 ("bpf: look into the types of the fields of a struct type recursively.")
Signed-off-by: Hou Tao <[e-mail]>
Acked-by: Eduard Zingerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/btf.c

···
  *   (i + 1) * elem_size
  * where i is the repeat index and elem_size is the size of an element.
  */
-static int btf_repeat_fields(struct btf_field_info *info,
+static int btf_repeat_fields(struct btf_field_info *info, int info_cnt,
 			     u32 field_cnt, u32 repeat_cnt, u32 elem_size)
 {
 	u32 i, j;
···
 			return -EINVAL;
 		}
 	}
+
+	/* The type of struct size or variable size is u32,
+	 * so the multiplication will not overflow.
+	 */
+	if (field_cnt * (repeat_cnt + 1) > info_cnt)
+		return -E2BIG;
 
 	cur = field_cnt;
 	for (i = 0; i < repeat_cnt; i++) {
···
 		info[i].off += off;
 
 	if (nelems > 1) {
-		err = btf_repeat_fields(info, ret, nelems - 1, t->size);
+		err = btf_repeat_fields(info, info_cnt, ret, nelems - 1, t->size);
 		if (err == 0)
 			ret *= nelems;
 		else
···
 
 	if (ret == BTF_FIELD_IGNORE)
 		return 0;
-	if (nelems > info_cnt)
+	if (!info_cnt)
 		return -E2BIG;
 	if (nelems > 1) {
-		ret = btf_repeat_fields(info, 1, nelems - 1, sz);
+		ret = btf_repeat_fields(info, info_cnt, 1, nelems - 1, sz);
 		if (ret < 0)
 			return ret;
 	}