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Kernel: Switch to eagerly restoring x86 FPU state on context switch

Lazy FPU restore is well known to be vulnerable to timing attacks,
and eager restore is a lot simpler anyway, so let's just do it eagerly.
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awesomekling committed Jan 1, 2020
1 parent 9c0836c commit fd740829d1976c7da2372b60e36d1ae569d6148d
Showing with 15 additions and 39 deletions.
  1. +3 −26 Kernel/Arch/i386/CPU.cpp
  2. +5 −2 Kernel/Scheduler.cpp
  3. +0 −1 Kernel/Scheduler.h
  4. +7 −7 Kernel/Thread.cpp
  5. +0 −3 Kernel/Thread.h
@@ -205,34 +205,11 @@ void general_protection_fault_handler(RegisterDump regs)

// 7: FPU not available exception
EH_ENTRY_NO_CODE(7, fpu_exception);
void fpu_exception_handler(RegisterDump regs)
void fpu_exception_handler(RegisterDump)
{
(void)regs;

// Just clear the TS flag. We've already restored the FPU state eagerly.
// FIXME: It would be nice if we didn't have to do this at all.
asm volatile("clts");
if (g_last_fpu_thread == current)
return;
if (g_last_fpu_thread) {
asm volatile("fxsave %0"
: "=m"(g_last_fpu_thread->fpu_state()));
} else {
asm volatile("fnclex");
}
g_last_fpu_thread = current;

if (current->has_used_fpu()) {
asm volatile("fxrstor %0" ::"m"(current->fpu_state()));
} else {
asm volatile("fninit");
asm volatile("fxsave %0"
: "=m"(current->fpu_state()));
current->set_has_used_fpu(true);
}

#ifdef FPU_EXCEPTION_DEBUG
kprintf("%s FPU not available exception: %u(%s)\n", current->process().is_ring0() ? "Kernel" : "Process", current->pid(), current->process().name().characters());
dump(regs);
#endif
}

// 14: Page Fault
@@ -39,7 +39,6 @@ static u32 time_slice_for(const Thread& thread)
}

Thread* current;
Thread* g_last_fpu_thread;
Thread* g_finalizer;
Thread* g_colonel;
WaitQueue* g_finalizer_wait_queue;
@@ -376,7 +375,6 @@ bool Scheduler::pick_next()
}
}


if (!thread_to_schedule)
thread_to_schedule = g_colonel;

@@ -457,6 +455,9 @@ bool Scheduler::context_switch(Thread& thread)
if (current->state() == Thread::Running)
current->set_state(Thread::Runnable);

asm volatile("fxsave %0"
: "=m"(current->fpu_state()));

#ifdef LOG_EVERY_CONTEXT_SWITCH
dbgprintf("Scheduler: %s(%u:%u) -> %s(%u:%u) [%u] %w:%x\n",
current->process().name().characters(), current->process().pid(), current->tid(),
@@ -469,6 +470,8 @@ bool Scheduler::context_switch(Thread& thread)
current = &thread;
thread.set_state(Thread::Running);

asm volatile("fxrstor %0" ::"m"(current->fpu_state()));

if (!thread.selector()) {
thread.set_selector(gdt_alloc_entry());
auto& descriptor = get_gdt_entry(thread.selector());
@@ -12,7 +12,6 @@ struct RegisterDump;
struct SchedulerData;

extern Thread* current;
extern Thread* g_last_fpu_thread;
extern Thread* g_finalizer;
extern Thread* g_colonel;
extern WaitQueue* g_finalizer_wait_queue;
@@ -10,6 +10,8 @@

//#define SIGNAL_DEBUG

static FPUState s_clean_fpu_state;

u16 thread_specific_selector()
{
static u16 selector;
@@ -55,7 +57,7 @@ Thread::Thread(Process& process)
dbgprintf("Thread{%p}: New thread TID=%u in %s(%u)\n", this, m_tid, process.name().characters(), process.pid());
set_default_signal_dispositions();
m_fpu_state = (FPUState*)kmalloc_aligned(sizeof(FPUState), 16);
memset(m_fpu_state, 0, sizeof(FPUState));
memcpy(m_fpu_state, &s_clean_fpu_state, sizeof(FPUState));
memset(&m_tss, 0, sizeof(m_tss));

// Only IF is set when a process boots.
@@ -119,9 +121,6 @@ Thread::~Thread()
thread_table().remove(this);
}

if (g_last_fpu_thread == this)
g_last_fpu_thread = nullptr;

if (selector())
gdt_free_entry(selector());

@@ -625,8 +624,7 @@ u32 Thread::make_userspace_stack_for_main_thread(Vector<String> arguments, Vecto
}
env[environment.size()] = nullptr;

auto push_on_new_stack = [&new_esp](u32 value)
{
auto push_on_new_stack = [&new_esp](u32 value) {
new_esp -= 4;
u32* stack_ptr = (u32*)new_esp;
*stack_ptr = value;
@@ -646,14 +644,16 @@ Thread* Thread::clone(Process& process)
memcpy(clone->m_signal_action_data, m_signal_action_data, sizeof(m_signal_action_data));
clone->m_signal_mask = m_signal_mask;
memcpy(clone->m_fpu_state, m_fpu_state, sizeof(FPUState));
clone->m_has_used_fpu = m_has_used_fpu;
clone->m_thread_specific_data = m_thread_specific_data;
return clone;
}

void Thread::initialize()
{
Scheduler::initialize();
asm volatile("fninit");
asm volatile("fxsave %0"
: "=m"(s_clean_fpu_state));
}

Vector<Thread*> Thread::all_threads()
@@ -329,8 +329,6 @@ class Thread {
bool has_signal_handler(u8 signal) const;

FPUState& fpu_state() { return *m_fpu_state; }
bool has_used_fpu() const { return m_has_used_fpu; }
void set_has_used_fpu(bool b) { m_has_used_fpu = b; }

void set_default_signal_dispositions();
void push_value_on_stack(u32);
@@ -457,7 +455,6 @@ class Thread {
u32 m_priority { THREAD_PRIORITY_NORMAL };
u32 m_extra_priority { 0 };
u32 m_priority_boost { 0 };
bool m_has_used_fpu { false };
bool m_dump_backtrace_on_finalization { false };
bool m_should_die { false };

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