Re: [PATCH v3 2/6] x86/entry_64: Add VERW just before userspace transition

[Pawan Gupta]

On Thu, Oct 26, 2023 at 12:40:49PM -0700, Dave Hansen wrote:
> On 10/26/23 12:29, Pawan Gupta wrote:
> > On Thu, Oct 26, 2023 at 07:25:27PM +0300, Nikolay Borisov wrote:
> >> On 25.10.23 г. 23:52 ч., Pawan Gupta wrote:
> >>> @@ -1520,6 +1530,7 @@ SYM_CODE_START(ignore_sysret)
> >>>   	UNWIND_HINT_END_OF_STACK
> >>>   	ENDBR
> >>>   	mov	$-ENOSYS, %eax
> >>> +	CLEAR_CPU_BUFFERS
> >> nit: Just out of curiosity is it really needed in this case or it's doesn
> >> for the sake of uniformity so that all ring3 transitions are indeed
> >> covered??
> > Interrupts returning to kernel don't clear the CPU buffers. I believe
> > interrupts will be enabled here, and getting an interrupt here could
> > leak the data that interrupt touched.
> 
> Specifically NMIs, right?

Yes, and VERW can omitted for the same reason as NMI returning to
kernel.

> X86_EFLAGS_IF should be clear here.

I see that SYSCALL has a configuration for IF, but I didn't see it for
SYSENTER in the code. But looking at the SDM, it clear IF by default.

syscall_init()
{
...
#else
	wrmsrl_cstar((unsigned long)ignore_sysret);
	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)GDT_ENTRY_INVALID_SEG);
	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, 0ULL);
#endif

	/*
	 * Flags to clear on syscall; clear as much as possible
	 * to minimize user space-kernel interference.
	 */
	wrmsrl(MSR_SYSCALL_MASK,
	       X86_EFLAGS_CF|X86_EFLAGS_PF|X86_EFLAGS_AF|
	       X86_EFLAGS_ZF|X86_EFLAGS_SF|X86_EFLAGS_TF|
	       X86_EFLAGS_IF|X86_EFLAGS_DF|X86_EFLAGS_OF|
	       X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_RF|
	       X86_EFLAGS_AC|X86_EFLAGS_ID);