Re: [PATCH v2 1/4] powerpc/64s: implement probe_kernel_read/write without touching AMR

[Daniel Borkmann]

Hey Nicholas,

On 4/7/20 6:01 AM, Nicholas Piggin wrote:
> Nicholas Piggin's on April 3, 2020 9:05 pm:
> > Christophe Leroy's on April 3, 2020 8:31 pm:
> > > Le 03/04/2020 Ã 11:35, Nicholas Piggin a ÃcritÂ:
> > > > There is no need to allow user accesses when probing kernel addresses.
> > >
> > > I just discovered the following commit
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=75a1a607bb7e6d918be3aca11ec2214a275392f4
> > >
> > > This commit adds probe_kernel_read_strict() and probe_kernel_write_strict().
> > >
> > > When reading the commit log, I understand that probe_kernel_read() may
> > > be used to access some user memory. Which will not work anymore with
> > > your patch.
> >
> > Hmm, I looked at _strict but obviously not hard enough. Good catch.
> >
> > I don't think probe_kernel_read() should ever access user memory,
> > the comment certainly says it doesn't, but that patch sort of implies
> > that they do.
> >
> > I think it's wrong. The non-_strict maybe could return userspace data to
> > you if you did pass a user address? I don't see why that shouldn't just
> > be disallowed always though.
> >
> > And if the _strict version is required to be safe, then it seems like a
> > bug or security issue to just allow everyone that doesn't explicitly
> > override it to use the default implementation.
> >
> > Also, the way the weak linkage is done in that patch, means parisc and
> > um archs that were previously overriding probe_kernel_read() now get
> > the default probe_kernel_read_strict(), which would be wrong for them.
>
> The changelog in commit 75a1a607bb7 makes it a bit clearer. If the
> non-_strict variant is used on non-kernel addresses, then it might not
> return -EFAULT or it might cause a kernel warning. The _strict variant
> is supposed to be usable with any address and it will return -EFAULT if
> it was not a valid and mapped kernel address.
>
> The non-_strict variant can not portably access user memory because it
> uses KERNEL_DS, and its documentation says its only for kernel pointers.
> So powerpc should be fine to run that under KUAP AFAIKS.
>
> I don't know why the _strict behaviour is not just made default, but
> the implementation of it does seem to be broken on the archs that
> override the non-_strict variant.

Yeah, we should make it default and only add a "opt out" for the old legacy
cases; there was also same discussion started over here just recently [0].

Thanks,
Daniel

  [0] https://lore.kernel.org/lkml/20200403133533.GA3424@xxxxxxxxxxxxx/T/