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

[Sean Christopherson]

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().