Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get_user paths

[Al Viro]

On Tue, Jan 16, 2018 at 08:30:17PM -0800, Dan Williams wrote:
> On Tue, Jan 16, 2018 at 2:23 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > On Sat, Jan 13, 2018 at 11:33 AM, Linus Torvalds
> [..]
> > I'll respin this set along those lines, and drop the ifence bits.
> 
> So now I'm not so sure. Yes, get_user_{1,2,4,8} can mask the pointer
> with the address limit result, but this doesn't work for the
> access_ok() + __get_user() case. We can either change the access_ok()
> calling convention to return a properly masked pointer to be used in
> subsequent calls to __get_user(), or go with lfence on every
> __get_user call. There seem to be several drivers that open code
> copy_from_user() with __get_user loops, so the 'fence every
> __get_user' approach might have noticeable overhead. On the other hand
> the access_ok conversion, while it could be scripted with coccinelle,
> is ~300 sites (VERIFY_READ), if you're concerned about having
> something small to merge for 4.15.
> 
> I think the access_ok() conversion to return a speculation sanitized
> pointer or NULL is the way to go unless I'm missing something simpler.
> Other ideas?

What masked pointer?  access_ok() exists for other architectures as well,
and the fewer callers remain outside of arch/*, the better.

Anything that open-codes copy_from_user() that way is *ALREADY* fucked if
it cares about the overhead - recent x86 boxen will have slowdown from
hell on stac()/clac() pairs.  Anything like that on a hot path is already
deep in trouble and needs to be found and fixed.  What drivers would those
be?  We don't have that many __get_user() users left outside of arch/*
anymore...