Kernel: Make copy_to/from_user safe and remove unnecessary checks

Since the CPU already does almost all necessary validation steps
for us, we don't really need to attempt to do this. Doing it
ourselves doesn't really work very reliably, because we'd have to
account for other processors modifying virtual memory, and we'd
have to account for e.g. pages not being able to be allocated
due to insufficient resources.

So change the copy_to/from_user (and associated helper functions)
to use the new safe_memcpy, which will return whether it succeeded
or not. The only manual validation step needed (which the CPU
can't perform for us) is making sure the pointers provided by user
mode aren't pointing to kernel mappings.

To make it easier to read/write from/to either kernel or user mode
data add the UserOrKernelBuffer helper class, which will internally
either use copy_from/to_user or directly memcpy, or pass the data
through directly using a temporary buffer on the stack.

Last but not least we need to keep syscall params trivial as we
need to copy them from/to user mode using copy_from/to_user.

1 files changed, 4 insertions, 12 deletions

diff --git a/Kernel/Syscalls/waitid.cpp b/Kernel/Syscalls/waitid.cpp
index 41a0d7c12a..1b12c2a744 100644
--- a/Kernel/Syscalls/waitid.cpp
+++ b/Kernel/Syscalls/waitid.cpp
@@ -104,27 +104,19 @@ pid_t Process::sys$waitid(Userspace<const Syscall::SC_waitid_params*> user_param
     REQUIRE_PROMISE(proc);
 
     Syscall::SC_waitid_params params;
-    if (!validate_read_and_copy_typed(&params, user_params))
-        return -EFAULT;
-
-    if (!validate_write_typed(params.infop))
+    if (!copy_from_user(&params, user_params))
         return -EFAULT;
 
 #ifdef PROCESS_DEBUG
-    dbg() << "sys$waitid(" << params.idtype << ", " << params.id << ", " << params.infop.ptr() << ", " << params.options << ")";
+    dbg() << "sys$waitid(" << params.idtype << ", " << params.id << ", " << params.infop << ", " << params.options << ")";
 #endif
 
     auto siginfo_or_error = do_waitid(static_cast<idtype_t>(params.idtype), params.id, params.options);
     if (siginfo_or_error.is_error())
         return siginfo_or_error.error();
-    // While we waited, the process lock was dropped. This gave other threads
-    // the opportunity to mess with the memory. For example, it could free the
-    // region, and map it to a region to which it has no write permissions.
-    // Therefore, we need to re-validate the pointer.
-    if (!validate_write_typed(params.infop))
-        return -EFAULT;
 
-    copy_to_user(params.infop, &siginfo_or_error.value());
+    if (!copy_to_user(params.infop, &siginfo_or_error.value()))
+        return -EFAULT;
     return 0;
 }