Re: [PATCH] x86: fix user address masking non-canonical speculation issue

[Kirill A. Shutemov]

On Wed, Oct 23, 2024 at 11:13:00PM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 23, 2024 at 06:31:59PM -0700, Linus Torvalds wrote:
> > @@ -2389,6 +2390,15 @@ void __init arch_cpu_finalize_init(void)
> > +		/*
> > +		 * Enable this when LAM is gated on LASS support
> > +		if (cpu_feature_enabled(X86_FEATURE_LAM))
> > +			USER_PTR_MAX = (1ul << 63) - PAGE_SIZE;
> > +		 */
> 
> I'm probably missing something but once LAM is enabled, how wouldn't
> this allow non-canonical address speculation?  i.e. when bit 47/56 is
> set and 63 is cleared, would it not go untouched by mask_user_address()
> and thus be speculatively interpreted by AMD as a kernel address?

CPU with LAM enabled enforces bit 63 to be equal bit 47/56 and raises #GP
otherwise.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov