ksud: migrate ksud execution to security_bprm_check (tiann#2653)

This migrates ksud execution decision-making to bprm_check_security.
This requires passing proper argv and envp to a modified _ksud handler
aptly named 'ksu_handle_bprm_ksud'.

Introduces:
int ksu_handle_bprm_ksud(const char *filename, const char *argv1,
const char *envp, size_t envp_len)

which is adapted from:
int ksu_handle_execveat_ksud(int *fd, struct filename **filename_ptr,
struct user_arg_ptr *argv,
struct user_arg_ptr *envp,
int *flags)

ksu_handle_bprm_ksud handles all the decision making, it decides when it is
time to apply_kernelsu_rules depending if it sees "second_stage".

For LSM hook, turns out we can pull out argv and envp from mm_struct.
The code in here explains itself on how to do it.

whole blob exists on arg_start to arg_end, so we just pull it out and grab next
array after the first null terminator.

as for envp, we pass the pointer then hunt for it when needed

My reasoning on adding a fallback on usercopy is that on some devices a fault
happens, and it copies garbled data. On my creation of this, I actually had to lock
that _nofault copy on a spinlock as a way to mimic preempt_disable/enable without
actually doing it. As per user reports, no failed _nofault copies anyway but we
have-to-have a fallback for resilience.

References:
- old version1 6efcd8193e
- old version2 37d5938e66
- bad usercopy #21

This now provides a small helper function, ksu_copy_from_user_retry, which explains
itself. First we attempt a _nofault copy, if that fails, we try plain.

With that, It also provides an inlined copy_from_user_nofault for < 5.8.

While using strncpy_from_user_nofault was considered, this wont do, this will
only copy up to the first \0.

devlog:
16e5dce9e7...16c1f5f521
28642e60d7...728de0c571

References:
https://elixir.bootlin.com/linux/v4.14.1/source/include/linux/mm_types.h#L429
https://elixir.bootlin.com/linux/v4.14.1/source/include/linux/lsm_hooks.h

Stale: https://github.com/tiann/KernelSU/pull/2653

Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

kernel/core_hook.c

@@ -21,6 +21,7 @@
 #include <linux/uidgid.h>
 #include <linux/version.h>
 #include <linux/mount.h>
+#include <linux/binfmts.h>
 
 #include <linux/fs.h>
 #include <linux/namei.h>
@@ -827,6 +828,19 @@ __maybe_unused int ksu_kprobe_exit(void)
 	return 0;
 }
 
+int ksu_bprm_check(struct linux_binprm *bprm)
+{
+	char *filename = (char *)bprm->filename;
+	
+	if (likely(!ksu_execveat_hook))
+		return 0;
+
+	ksu_handle_pre_ksud(filename);
+
+	return 0;
+
+}
+
 static int ksu_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			  unsigned long arg4, unsigned long arg5)
 {
@@ -851,6 +865,9 @@ static struct security_hook_list ksu_hooks[] = {
 	LSM_HOOK_INIT(task_prctl, ksu_task_prctl),
 	LSM_HOOK_INIT(inode_rename, ksu_inode_rename),
 	LSM_HOOK_INIT(task_fix_setuid, ksu_task_fix_setuid),
+#ifndef CONFIG_KSU_KPROBES_HOOK
+	LSM_HOOK_INIT(bprm_check_security, ksu_bprm_check),
+#endif
 };
 
 void __init ksu_lsm_hook_init(void)