Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

[Jason Wang]

On 2020/6/2 äå4:45, Michael S. Tsirkin wrote:
> So vhost needs to poke at userspace *a lot* in a quick succession.  It
> is thus benefitial to enable userspace access, do our thing, then
> disable. Except access_ok has already been pre-validated with all the
> relevant nospec checks, so we don't need that.  Add an API to allow
> userspace access after access_ok and barrier_nospec are done.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>
> Jason, so I've been thinking using something along these lines,
> then switching vhost to use unsafe_copy_to_user and friends would
> solve lots of problems you observed with SMAP.
>
> What do you think?


I'm fine with this approach.


>   Do we need any other APIs to make it practical?


It's not clear whether we need a new API, I think __uaccess_being() has 
the assumption that the address has been validated by access_ok().

Thanks


>   arch/x86/include/asm/uaccess.h | 1 +
>   include/linux/uaccess.h        | 1 +
>   2 files changed, 2 insertions(+)
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index d8f283b9a569..fa5afb3a54fe 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -483,6 +483,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
>   	return 1;
>   }
>   #define user_access_begin(a,b)	user_access_begin(a,b)
> +#define user_access_begin_after_access_ok()	__uaccess_begin()
>   #define user_access_end()	__uaccess_end()
>   
>   #define user_access_save()	smap_save()
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 67f016010aad..4c0a959ad639 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -370,6 +370,7 @@ extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);
>   
>   #ifndef user_access_begin
>   #define user_access_begin(ptr,len) access_ok(ptr, len)
> +#define user_access_begin_after_access_ok() do { } while (0)
>   #define user_access_end() do { } while (0)
>   #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
>   #define unsafe_get_user(x,p,e) unsafe_op_wrap(__get_user(x,p),e)