Kernel: Rework syscall memory validation and locking

Process's memory regions are now behind an rwlock instead of using the
full process lock. This allows most pointer validations to not block as
write operations to memory regions are rare.

Thread's userspace stack is now part of process's memory regions. This
simplifies code that explicitly looped over threads to see if the
accessed address was inside a thread's stack.

Only drawback of this is that MemoryRegions don't support guard pages,
so userspace stackoverflow will be handeled as cleanly as it was prior
to this.

This patch also fixes some unnecessary locking of the process lock and
moves locking to the internal helper functions instead of asserting that
the lock is held. Also we now make sure loaded ELF regions are in sorted
order as we previously expected.

kernel/include/kernel/Process.h

@@ -9,6 +9,7 @@
 #include <kernel/ELF.h>
 #include <kernel/FS/Inode.h>
 #include <kernel/Lock/Mutex.h>
+#include <kernel/Lock/RWLock.h>
 #include <kernel/Memory/Heap.h>
 #include <kernel/Memory/MemoryRegion.h>
 #include <kernel/Memory/SharedMemoryObject.h>
@@ -272,17 +273,20 @@ namespace Kernel
 		};
 
 		// Adds new region to the sorted array of mapped regions
+		// You must hold writer end of m_mapped_region_lock when calling this.
 		BAN::ErrorOr<void> add_mapped_region(BAN::UniqPtr<MemoryRegion>&&);
 		// If address is contained by a region, returns the index of that.
 		// Otherwise returns the address of the first region after this address.
+		// You must hold reader end of m_mapped_region_lock when calling this.
 		size_t find_mapped_region(vaddr_t) const;
 
 		BAN::ErrorOr<VirtualFileSystem::File> find_file(int fd, const char* path, int flags) const;
 		BAN::ErrorOr<FileParent> find_parent_file(int fd, const char* path, int flags) const;
 		BAN::ErrorOr<VirtualFileSystem::File> find_relative_parent(int fd, const char* path) const;
 
-		BAN::ErrorOr<void> validate_string_access(const char*);
-		BAN::ErrorOr<void> validate_pointer_access(const void*, size_t, bool needs_write);
+		BAN::ErrorOr<void> read_from_user(const void* user_addr, void* out, size_t size);
+		BAN::ErrorOr<void> read_string_from_user(const char* user_addr, char* out, size_t max_size);
+		BAN::ErrorOr<void> write_to_user(void* user_addr, const void* in, size_t size);
 		BAN::ErrorOr<MemoryRegion*> validate_and_pin_pointer_access(const void*, size_t, bool needs_write);
 
 		uint64_t signal_pending_mask() const
@@ -327,6 +331,7 @@ namespace Kernel
 
 		OpenFileDescriptorSet m_open_file_descriptors;
 
+		mutable RWLock m_memory_region_lock;
 		BAN::Vector<BAN::UniqPtr<MemoryRegion>> m_mapped_regions;
 
 		pid_t m_sid;

kernel/include/kernel/Thread.h

@@ -16,6 +16,7 @@
 namespace Kernel
 {
 
+	class MemoryBackedRegion;
 	class Process;
 
 	class Thread
@@ -103,9 +104,7 @@ namespace Kernel
 		vaddr_t kernel_stack_top() const	{ return m_kernel_stack->vaddr() + m_kernel_stack->size(); }
 		VirtualRange& kernel_stack() { return *m_kernel_stack; }
 
-		vaddr_t userspace_stack_bottom() const	{ return is_userspace() ? m_userspace_stack->vaddr() : UINTPTR_MAX; }
-		vaddr_t userspace_stack_top() const		{ return is_userspace() ? m_userspace_stack->vaddr() + m_userspace_stack->size() : 0; }
-		VirtualRange& userspace_stack() { ASSERT(is_userspace()); return *m_userspace_stack; }
+		MemoryBackedRegion& userspace_stack() { ASSERT(is_userspace() && m_userspace_stack); return *m_userspace_stack; }
 
 		static Thread& current();
 		static pid_t current_tid();
@@ -129,7 +128,7 @@ namespace Kernel
 		void set_gsbase(vaddr_t base) { m_gsbase = base; }
 		vaddr_t get_gsbase() const { return m_gsbase; }
 
-		size_t virtual_page_count() const { return (m_kernel_stack ? (m_kernel_stack->size() / PAGE_SIZE) : 0) + (m_userspace_stack ? (m_userspace_stack->size() / PAGE_SIZE) : 0); }
+		size_t virtual_page_count() const { return m_kernel_stack ? (m_kernel_stack->size() / PAGE_SIZE) : 0; }
 		size_t physical_page_count() const { return virtual_page_count(); }
 
 		YieldRegisters& yield_registers() { return m_yield_registers; }
@@ -160,7 +159,7 @@ namespace Kernel
 		BAN::UniqPtr<PageTable>    m_keep_alive_page_table;
 
 		BAN::UniqPtr<VirtualRange> m_kernel_stack;
-		BAN::UniqPtr<VirtualRange> m_userspace_stack;
+		MemoryBackedRegion*        m_userspace_stack      { nullptr };
 		const pid_t                m_tid                  { 0 };
 		State                      m_state                { State::NotStarted };
 		Process*                   m_process              { nullptr };

kernel/kernel/Memory/VirtualRange.cpp

@@ -148,12 +148,12 @@ namespace Kernel
 		if (m_preallocated)
 			return false;
 
+		SpinLockGuard _(m_lock);
+
 		const size_t index = (vaddr - this->vaddr()) / PAGE_SIZE;
 		if (m_paddrs[index])
 			return false;
 
-		SpinLockGuard _(m_lock);
-
 		m_paddrs[index] = Heap::get().take_free_page();
 		if (m_paddrs[index] == 0)
 			return BAN::Error::from_errno(ENOMEM);

kernel/kernel/Thread.cpp

@@ -4,6 +4,7 @@
 #include <kernel/InterruptController.h>
 #include <kernel/InterruptStack.h>
 #include <kernel/Memory/kmalloc.h>
+#include <kernel/Memory/MemoryBackedRegion.h>
 #include <kernel/Process.h>
 #include <kernel/Scheduler.h>
 #include <kernel/Thread.h>
@@ -218,13 +219,16 @@ namespace Kernel
 			true, true
 		));
 
-		thread->m_userspace_stack = TRY(VirtualRange::create_to_vaddr_range(
+		auto userspace_stack = TRY(MemoryBackedRegion::create(
 			page_table,
-			stack_addr_start, USERSPACE_END,
 			userspace_stack_size,
+			{ stack_addr_start, USERSPACE_END },
+			MemoryRegion::Type::PRIVATE,
 			PageTable::Flags::UserSupervisor | PageTable::Flags::ReadWrite | PageTable::Flags::Present,
-			false, true
+			O_RDWR
 		));
+		thread->m_userspace_stack = userspace_stack.ptr();
+		TRY(process->add_mapped_region(BAN::move(userspace_stack)));
 
 		thread_deleter.disable();
 
@@ -319,16 +323,18 @@ namespace Kernel
 	{
 		auto* thread = TRY(create_userspace(m_process, m_process->page_table()));
 
+		if (Processor::get_current_sse_thread() == this)
 			save_sse();
 		memcpy(thread->m_sse_storage, m_sse_storage, sizeof(m_sse_storage));
 
-		TRY(thread->userspace_stack().allocate_page_for_demand_paging(thread->userspace_stack_top() - PAGE_SIZE));
-		PageTable::with_fast_page(thread->userspace_stack().paddr_of(thread->userspace_stack_top() - PAGE_SIZE), [=] {
-			PageTable::fast_page_as<void*>(PAGE_SIZE - sizeof(uintptr_t)) = arg;
-		});
+		TRY(thread->userspace_stack().copy_data_to_region(
+			thread->m_userspace_stack->size() - sizeof(void*),
+			reinterpret_cast<const uint8_t*>(&arg),
+			sizeof(void*)
+		));
 
 		const vaddr_t entry_addr = reinterpret_cast<vaddr_t>(entry);
-		thread->setup_exec(entry_addr, thread->userspace_stack_top() - sizeof(uintptr_t));
+		thread->setup_exec(entry_addr, thread->userspace_stack().vaddr() + thread->userspace_stack().size() - sizeof(void*));
 
 		return thread;
 	}
@@ -346,13 +352,17 @@ namespace Kernel
 		thread->m_is_userspace = true;
 
 		thread->m_kernel_stack = TRY(m_kernel_stack->clone(new_process->page_table()));
-		thread->m_userspace_stack = TRY(m_userspace_stack->clone(new_process->page_table()));
+
+		const auto stack_index = new_process->find_mapped_region(m_userspace_stack->vaddr());
+		thread->m_userspace_stack = static_cast<MemoryBackedRegion*>(new_process->m_mapped_regions[stack_index].ptr());
+		ASSERT(thread->m_userspace_stack->vaddr() == m_userspace_stack->vaddr());
 
 		thread->m_fsbase = m_fsbase;
 		thread->m_gsbase = m_gsbase;
 
 		thread->m_state = State::NotStarted;
 
+		if (Processor::get_current_sse_thread() == this)
 			save_sse();
 		memcpy(thread->m_sse_storage, m_sse_storage, sizeof(m_sse_storage));
 
@@ -397,29 +407,18 @@ namespace Kernel
 		if (needed_size > m_userspace_stack->size())
 			return BAN::Error::from_errno(ENOBUFS);
 
-		vaddr_t vaddr = userspace_stack_top() - needed_size;
+		vaddr_t vaddr = userspace_stack().vaddr() + userspace_stack().size() - needed_size;
 
 		const size_t page_count = BAN::Math::div_round_up<size_t>(needed_size, PAGE_SIZE);
 		for (size_t i = 0; i < page_count; i++)
-			TRY(m_userspace_stack->allocate_page_for_demand_paging(vaddr + i * PAGE_SIZE));
+			TRY(m_userspace_stack->allocate_page_containing(vaddr + i * PAGE_SIZE, true));
 
 		const auto stack_copy_buf =
 			[this](BAN::ConstByteSpan buffer, vaddr_t vaddr) -> void
 			{
-				ASSERT(vaddr + buffer.size() <= userspace_stack_top());
-
-				size_t bytes_copied = 0;
-				while (bytes_copied < buffer.size())
-				{
-					const size_t to_copy = BAN::Math::min<size_t>(buffer.size() - bytes_copied, PAGE_SIZE - (vaddr % PAGE_SIZE));
-
-					PageTable::with_fast_page(userspace_stack().paddr_of(vaddr & PAGE_ADDR_MASK), [=]() {
-						memcpy(PageTable::fast_page_as_ptr(vaddr % PAGE_SIZE), buffer.data() + bytes_copied, to_copy);
-					});
-
-					vaddr += to_copy;
-					bytes_copied += to_copy;
-				}
+				ASSERT(vaddr >= m_userspace_stack->vaddr());
+				ASSERT(vaddr + buffer.size() <= m_userspace_stack->vaddr() + m_userspace_stack->size());
+				MUST(m_userspace_stack->copy_data_to_region(vaddr - m_userspace_stack->vaddr(), buffer.data(), buffer.size()));
 			};
 
 		const auto stack_push_buf =
@@ -471,7 +470,7 @@ namespace Kernel
 			stack_push_str(envp[i]);
 		}
 
-		setup_exec(entry, userspace_stack_top() - needed_size);
+		setup_exec(entry, m_userspace_stack->vaddr() + m_userspace_stack->size() - needed_size);
 
 		return {};
 	}