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/kernel_compat.h

@@ -60,5 +60,21 @@ extern ssize_t ksu_kernel_read_compat(struct file *p, void *buf, size_t count,
 				      loff_t *pos);
 extern ssize_t ksu_kernel_write_compat(struct file *p, const void *buf,
 				       size_t count, loff_t *pos);
+/*
+ * ksu_copy_from_user_retry
+ * try nofault copy first, if it fails, try with plain
+ * paramters are the same as copy_from_user
+ * 0 = success
+ */
+static long ksu_copy_from_user_retry(void *to, 
+		const void __user *from, unsigned long count)
+{
+	long ret = copy_from_user_nofault(to, from, count);
+	if (likely(!ret))
+		return ret;
+
+	// we faulted! fallback to slow path
+	return copy_from_user(to, from, count);
+}
 
 #endif