[PATCH] x86/uaccess: Avoid barrier_nospec() in copy_from_user()

[Josh Poimboeuf]

On Sat, Oct 12, 2024 at 09:48:57AM +0100, Andrew Cooper wrote:
> On 12/10/2024 5:09 am, Josh Poimboeuf wrote:
> > For x86-64, the barrier_nospec() in copy_from_user() is overkill and
> > painfully slow.  Instead, use pointer masking to force the user pointer
> > to a non-kernel value even in speculative paths.
> >
> > Signed-off-by: Josh Poimboeuf <jpoimboe at kernel.org>
> 
> You do realise mask_user_address() is unsafe under speculation on AMD
> systems?
> 
> Had the mask_user_address() patch been put for review, this feedback
> would have been given then.
> 
> 
> AMD needs to arrange for bit 47 (bit 58 with LA57) to be the one
> saturated by shifting, not bit 63.

Ok... why?

-- 
Josh