From df257755f7a0bc43eeb8061e5b7c1504c6c772d1 Mon Sep 17 00:00:00 2001
From: Bananymous <oskari.alaranta@bananymous.com>
Date: Sat, 4 Apr 2026 23:35:33 +0300
Subject: [PATCH] Kernel: If userspace sets fs or gs, dont overwrite it

Current cpu index is stored at either segment. If userspace sets that
segment, kernel will not overwrite it on every reschedule. This is fine
as long as user program does not use anything that relies on it :)
---
 kernel/include/kernel/Thread.h | 2 ++
 kernel/kernel/Process.cpp      | 8 ++++++--
 kernel/kernel/Thread.cpp       | 8 ++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/include/kernel/Thread.h b/kernel/include/kernel/Thread.h
index 9c3c9e4f..42d094aa 100644
--- a/kernel/include/kernel/Thread.h
+++ b/kernel/include/kernel/Thread.h
@@ -177,7 +177,9 @@ namespace Kernel
 		bool                       m_is_userspace         { false };
 		bool                       m_delete_process       { false };
 
+		bool                       m_has_custom_fsbase    { false };
 		vaddr_t                    m_fsbase               { 0 };
+		bool                       m_has_custom_gsbase    { false };
 		vaddr_t                    m_gsbase               { 0 };
 
 		SchedulerQueue::Node*      m_scheduler_node       { nullptr };
diff --git a/kernel/kernel/Process.cpp b/kernel/kernel/Process.cpp
index 255991f3..eb0eb4ae 100644
--- a/kernel/kernel/Process.cpp
+++ b/kernel/kernel/Process.cpp
@@ -3302,7 +3302,9 @@ namespace Kernel
 
 	BAN::ErrorOr<long> Process::sys_set_fsbase(void* addr)
 	{
-		Thread::current().set_fsbase(reinterpret_cast<vaddr_t>(addr));
+		auto& thread = Thread::current();
+		thread.m_has_custom_fsbase = true;
+		thread.set_fsbase(reinterpret_cast<vaddr_t>(addr));
 		Processor::load_fsbase();
 		return 0;
 	}
@@ -3314,7 +3316,9 @@ namespace Kernel
 
 	BAN::ErrorOr<long> Process::sys_set_gsbase(void* addr)
 	{
-		Thread::current().set_gsbase(reinterpret_cast<vaddr_t>(addr));
+		auto& thread = Thread::current();
+		thread.m_has_custom_gsbase = true;
+		thread.set_gsbase(reinterpret_cast<vaddr_t>(addr));
 		Processor::load_gsbase();
 		return 0;
 	}
diff --git a/kernel/kernel/Thread.cpp b/kernel/kernel/Thread.cpp
index 2eafc640..485cceb6 100644
--- a/kernel/kernel/Thread.cpp
+++ b/kernel/kernel/Thread.cpp
@@ -305,6 +305,14 @@ namespace Kernel
 		if (!is_userspace() || !has_process())
 			return;
 
+#if ARCH(x86_64)
+		if (m_has_custom_gsbase)
+			return;
+#elif ARCH(i686)
+		if (m_has_custom_fsbase)
+			return;
+#endif
+
 		const vaddr_t vaddr = process().shared_page_vaddr() + Processor::current_index();
 
 #if ARCH(x86_64)