Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

Josh Poimboeuf
2021-06-02T20:11:25Z

On Wed, Jun 02, 2021 at 05:11:57PM +0000, Sean Christopherson wrote:
> On Tue, May 04, 2021, Josh Poimboeuf wrote:
> > The x86 uaccess code uses barrier_nospec() in various places to prevent
> > speculative dereferencing of user-controlled pointers (which might be
> > combined with further gadgets or CPU bugs to leak data).
> >
> > There are some issues with the current implementation:
> >
> > - The barrier_nospec() in copy_from_user() was inadvertently removed
> > with: 4b842e4e25b1 ("x86: get rid of small constant size cases in
> > raw_copy_{to,from}_user()")
>
> Mostly out of curiosity, wasn't copy_{from,to}_user() flawed even before that
> patch? Non-constant sizes would go straight to copy_user_generic(), and even if
> string ops are used and strings are magically not vulnerable, small sizes would
> skip to normal loads/stores in _copy_short_string when using
> copy_user_enhanced_fast_string().

Yes, it appears so.

--
Josh