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.

Kernel/FileSystem/ProcFS.cpp

@@ -976,21 +976,33 @@ static ByteBuffer read_sys_bool(InodeIdentifier inode_id)
     return buffer;
 }
 
-static ssize_t write_sys_bool(InodeIdentifier inode_id, const ByteBuffer& data)
+static ssize_t write_sys_bool(InodeIdentifier inode_id, const UserOrKernelBuffer& buffer, size_t size)
 {
     auto& variable = SysVariable::for_inode(inode_id);
     ASSERT(variable.type == SysVariable::Type::Boolean);
 
-    if (data.is_empty() || !(data[0] == '0' || data[0] == '1'))
-        return data.size();
+    char value = 0;
+    bool did_read = false;
+    ssize_t nread = buffer.read_buffered<1>(1, [&](const u8* data, size_t) {
+        if (did_read)
+            return 0;
+        value = (char)data[0];
+        did_read = true;
+        return 1;
+    });
+    if (nread < 0)
+        return nread;
+    ASSERT(nread == 0 || (nread == 1 && did_read));
+    if (nread == 0 || !(value == '0' || value == '1'))
+        return (ssize_t)size;
 
     auto* lockable_bool = reinterpret_cast<Lockable<bool>*>(variable.address);
     {
         LOCKER(lockable_bool->lock());
-        lockable_bool->resource() = data[0] == '1';
+        lockable_bool->resource() = value == '1';
     }
     variable.notify();
-    return data.size();
+    return (ssize_t)size;
 }
 
 static ByteBuffer read_sys_string(InodeIdentifier inode_id)
@@ -1003,18 +1015,22 @@ static ByteBuffer read_sys_string(InodeIdentifier inode_id)
     return lockable_string->resource().to_byte_buffer();
 }
 
-static ssize_t write_sys_string(InodeIdentifier inode_id, const ByteBuffer& data)
+static ssize_t write_sys_string(InodeIdentifier inode_id, const UserOrKernelBuffer& buffer, size_t size)
 {
     auto& variable = SysVariable::for_inode(inode_id);
     ASSERT(variable.type == SysVariable::Type::String);
 
+    auto string_copy = buffer.copy_into_string(size);
+    if (string_copy.is_null())
+        return -EFAULT;
+
     {
         auto* lockable_string = reinterpret_cast<Lockable<String>*>(variable.address);
         LOCKER(lockable_string->lock());
-        lockable_string->resource() = String((const char*)data.data(), data.size());
+        lockable_string->resource() = move(string_copy);
     }
     variable.notify();
-    return data.size();
+    return (ssize_t)size;
 }
 
 void ProcFS::add_sys_bool(String&& name, Lockable<bool>& var, Function<void()>&& notify_callback)
@@ -1180,13 +1196,13 @@ InodeMetadata ProcFSInode::metadata() const
     return metadata;
 }
 
-ssize_t ProcFSInode::read_bytes(off_t offset, ssize_t count, u8* buffer, FileDescription* description) const
+ssize_t ProcFSInode::read_bytes(off_t offset, ssize_t count, UserOrKernelBuffer& buffer, FileDescription* description) const
 {
 #ifdef PROCFS_DEBUG
     dbg() << "ProcFS: read_bytes " << index();
 #endif
     ASSERT(offset >= 0);
-    ASSERT(buffer);
+    ASSERT(buffer.user_or_kernel_ptr());
 
     auto* directory_entry = fs().get_directory_entry(identifier());
 
@@ -1240,7 +1256,8 @@ ssize_t ProcFSInode::read_bytes(off_t offset, ssize_t count, u8* buffer, FileDes
         return 0;
 
     ssize_t nread = min(static_cast<off_t>(data.value().size() - offset), static_cast<off_t>(count));
-    memcpy(buffer, data.value().data() + offset, nread);
+    if (!buffer.write(data.value().data() + offset, nread))
+        return -EFAULT;
     if (nread == 0 && description && description->generator_cache().has_value())
         description->generator_cache().clear();
 
@@ -1461,7 +1478,7 @@ void ProcFSInode::flush_metadata()
 {
 }
 
-ssize_t ProcFSInode::write_bytes(off_t offset, ssize_t size, const u8* buffer, FileDescription*)
+ssize_t ProcFSInode::write_bytes(off_t offset, ssize_t size, const UserOrKernelBuffer& buffer, FileDescription*)
 {
     auto result = prepare_to_write_data();
     if (result.is_error())
@@ -1469,8 +1486,8 @@ ssize_t ProcFSInode::write_bytes(off_t offset, ssize_t size, const u8* buffer, F
 
     auto* directory_entry = fs().get_directory_entry(identifier());
 
-    Function<ssize_t(InodeIdentifier, const ByteBuffer&)> callback_tmp;
-    Function<ssize_t(InodeIdentifier, const ByteBuffer&)>* write_callback { nullptr };
+    Function<ssize_t(InodeIdentifier, const UserOrKernelBuffer&, size_t)> callback_tmp;
+    Function<ssize_t(InodeIdentifier, const UserOrKernelBuffer&, size_t)>* write_callback { nullptr };
 
     if (directory_entry == nullptr) {
         if (to_proc_parent_directory(identifier()) == PDI_Root_sys) {
@@ -1496,9 +1513,10 @@ ssize_t ProcFSInode::write_bytes(off_t offset, ssize_t size, const u8* buffer, F
     ASSERT(is_persistent_inode(identifier()));
     // FIXME: Being able to write into ProcFS at a non-zero offset seems like something we should maybe support..
     ASSERT(offset == 0);
-    bool success = (*write_callback)(identifier(), ByteBuffer::wrap(const_cast<u8*>(buffer), size));
-    ASSERT(success);
-    return 0;
+    ssize_t nwritten = (*write_callback)(identifier(), buffer, (size_t)size);
+    if (nwritten < 0)
+        klog() << "ProcFS: Writing " << size << " bytes failed: " << nwritten;
+    return nwritten;
 }
 
 KResultOr<NonnullRefPtr<Custody>> ProcFSInode::resolve_as_link(Custody& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level) const