Re: [PATCH] x86/asm: add __user on copy_user_handle_tail() pointers

[Linus Torvalds]

On Thu, Mar 28, 2019 at 12:24 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> Well, but copy_user_generic() (which ends up calling the
> copy_user_handle_tail() eventually) casts those __user pointers to
> (__force void *). Converting them back to __user looks strange to me.
>
> Linus?

Well, it does that because the x86 version of copy_user_generic() can
work in either direction, so it works when either the source or
destination (or both) are user pointers, but they don't _have_ to be.

So the "userness" of a pointer in that context is a bit ambiguous, and
so we've picked the pointers to be just plain "void *".

That said, arguably we should have gone the other way and just made
them both "__user" pointers, and do the cast the other way around.

But there's no absolutely right answer here, and nobody should ever
use copy_user_generic() directly (ie it is very much meant to be only
used as a internal helper for the cases that get the pointer
annotations right).

I do think Ben's patch is probably the right thing to do.

And we could do the same thing to copy_user_generic(), but that would
require switching the casts around in the callers, so may not be worth
the noise.

                      Linus