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

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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.
This commit is contained in:
Andreas Kling 2020-01-01 16:49:08 +01:00
parent 9c0836ce97
commit fd740829d1
5 changed files with 15 additions and 39 deletions
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29
Kernel/Arch/i386/CPU.cpp
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@ -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