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

[Daniel Thompson]

On Wed, Jan 12, 2022 at 06:08:17PM +0000, Russell King (Oracle) wrote:
> On Wed, Jan 12, 2022 at 05:29:03PM +0000, Daniel Thompson wrote:
> > On Mon, Jul 26, 2021 at 04:11:39PM +0200, Arnd Bergmann wrote:
> > > From: Arnd Bergmann <arnd@xxxxxxxx>
> > > 
> > > These mimic the behavior of get_user and put_user, except
> > > for domain switching, address limit checking and handling
> > > of mismatched sizes, none of which are relevant here.
> > > 
> > > To work with pre-Armv6 kernels, this has to avoid TUSER()
> > > inside of the new macros, the new approach passes the "t"
> > > string along with the opcode, which is a bit uglier but
> > > avoids duplicating more code.
> > > 
> > > As there is no __get_user_asm_dword(), I work around it
> > > by copying 32 bit at a time, which is possible because
> > > the output size is known.
> > > 
> > > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > 
> > I've just been bisecting some regressions running the kgdbts tests on
> > arm and this patch came up.
> 
> So the software PAN code is working :)

Interesting. I noticed it was odd that kgdbts works just fine
if launched from kernel command line. I guess that runs before
PAN is activated. Neat.


> 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.


Daniel.