Re: [PULL] vhost: cleanups and fixes

[Al Viro]

On Fri, Nov 02, 2018 at 10:15:56AM -0700, Linus Torvalds wrote:
> On Fri, Nov 2, 2018 at 10:10 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Don't you take over the VM with "use_mm()" when you do the copies? So
> > yes, it's a kernel thread, but it has a user VM, and though that
> > should have the user limits.
> 
> Oooh. *Just* as I sent this, I realized that "use_mm()" doesn't update
> the thread addr_limit.
> 
> That actually looks like a bug to me - although one that you've
> apparently been aware of and worked around.
> 
> Wouldn't it be nicer to just make "use_mm()" do
> 
>         set_fs(USER_DS);
> 
> instead? And undo it on unuse_mm()?
> 
> And, in fact, maybe we should default kernel threads to have a zero
> address limit, so that they can't do any user accesses at all without
> doing this?

Try it and watch it fail to set initramfs up, let alone exec the init...

> Adding Al to the cc, because I think he's been looking at set_fs() in general.

It would be the right thing (with return to KERNEL_DS), but I'm not certain
if GPU users will survive - these two
drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.h:157:                         use_mm(mmptr);                          \
drivers/gpu/drm/i915/gvt/kvmgt.c:1799:          use_mm(kvm->mm);
I don't understand the call chains there (especially for the first one) well
enough to tell.