On Sat, Jan 6, 2018 at 5:20 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Jan 6, 2018 at 3:31 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>>
>> I assume if we put this in uaccess_begin() we also need audit for
>> paths that use access_ok but don't do on to call uaccess_begin()? A
>> quick glance shows a few places where we are open coding the stac().
>> Perhaps land the lfence in stac() directly?
>
> Yeah, we should put it in uaccess_begin(), and in the actual user
> accessor helpers that do stac. Some of them probably should be changed
> to use uaccess_begin() instead while at it.
>
> One question for the CPU people: do we actually care and need to do
> this for things that might *write* to something? The speculative write
> obviously is killed, but does it perhaps bring in a cacheline even
> when killed?
As far as I understand a write could trigger a request-for-ownership
read for the target cacheline.
> Because maybe we don't need the lfence in put_user(), only in get_user()?
Even though writes can trigger reads, as far as I can see the write
needs to be dependent on the first out-of-bounds read:
if (x < max)
y = array1[x];
put_user(array2 + y, z);
...in other words that first read should be annotated with
nospec_array_ptr() making an lfence in put_user() or other writes
moot.
yp = nospec_array_ptr(array1, x, max);
if (yp)
y = *yp;
put_user(array2 + y, z);
