Re: [PATCH v5 08/10] ARM: uaccess: add __{get,put}_kernel_nofault

[Arnd Bergmann]

On Thu, Jan 13, 2022 at 10:47 AM Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
> On Wed, Jan 12, 2022 at 06:08:17PM +0000, Russell King (Oracle) wrote:
>
> > The kernel attempted to access an address that is in the userspace
> > domain (NULL pointer) and took an exception.
> >
> > I suppose we should handle a domain fault more gracefully - what are
> > the required semantics if the kernel attempts a userspace access
> > using one of the _nofault() accessors?
>
> I think the best answer might well be that, if the arch provides
> implementations of hooks such as copy_from_kernel_nofault_allowed()
> then the kernel should never attempt a userspace access using the
> _nofault() accessors. That means they can do whatever they like!
>
> In other words something like the patch below looks like a promising
> approach.

Right, it seems this is the same as on x86.

> From f66a63b504ff582f261a506c54ceab8c0e77a98c Mon Sep 17 00:00:00 2001
> From: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> Date: Thu, 13 Jan 2022 09:34:45 +0000
> Subject: [PATCH] arm: mm: Implement copy_from_kernel_nofault_allowed()
>
> Currently copy_from_kernel_nofault() can actually fault (due to software
> PAN) if we attempt userspace access. In any case, the documented
> behaviour for this function is to return -ERANGE if we attempt an access
> outside of kernel space.
>
> Implementing copy_from_kernel_nofault_allowed() solves both these
> problems.
>
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>

Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>