Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

From: Linus Torvalds
Date: Fri Feb 22 2019 - 13:29:13 EST

On Fri, Feb 22, 2019 at 9:48 AM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Feb 22, 2019, at 9:43 AM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Then we should still probably fix up "__probe_kernel_read()" to not
> > allow user accesses. The easiest way to do that is actually likely to
> > use the "unsafe_get_user()" functions *without* doing a
> > uaccess_begin(), which will mean that modern CPU's will simply fault
> > on a kernel access to user space.
> >
> > The nice thing about that is that usually developers will have access
> > to exactly those modern boxes, so the people who notice that it
> > doesn't work are the right people.
> We use probe_kernel_read() from oops code. Iâd rather it return -EFAULT than oops harder and kill the first oops.

It would still do that.

Using the unsafe_get_user() macros doesn't remove the exception
handling, and we wouldn't remove the whole "pagefault_disable()"
either. So it would work exactly the same way it does now, except on a
modern CPU it would return -EFAULT for a user space access due to AC
not being set.

So no "oops harder", only "safer accesses".