Re: [PATCH] powerpc/mm: Fix KUAP warning by providing copy_from_kernel_nofault_allowed()

[Christophe Leroy]

Le 05/12/2020 à 10:56, Christophe Leroy a écrit :
> Le 05/12/2020 à 09:48, Christoph Hellwig a écrit :
> > On Sat, Dec 05, 2020 at 08:43:06AM +0000, Christophe Leroy wrote:
> > > Since commit c33165253492 ("powerpc: use non-set_fs based maccess
> > > routines"), userspace access is not granted anymore when using
> > > copy_from_kernel_nofault()
> > >
> > > However, kthread_probe_data() uses copy_from_kernel_nofault()
> > > to check validity of pointers. When the pointer is NULL,
> > > it points to userspace, leading to a KUAP fault and triggering
> > > the following big hammer warning many times when you request
> > > a sysrq "show task":
> >
> >
> >
> > > To avoid that, copy_from_kernel_nofault_allowed() is used to check
> > > whether the address is a valid kernel address. But the default
> > > version of it returns true for any address.
> > >
> > > Provide a powerpc version of copy_from_kernel_nofault_allowed()
> > > that returns false when the address is below TASK_USER_MAX,
> > > so that copy_from_kernel_nofault() will return -ERANGE.
> >
> > Looks good.  I wonder if we should just default to the TASK_SIZE_MAX
> > check in  copy_from_kernel_nofault_allowed for architectures that select
> > CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE?
>
> Yes maybe that would be better.
>
> Can you cook a patch an get it into 5.10 ?

In fact it doesn't seem so easy because only s390, powerpc and x86 have TASK_SIZE_MAX while 
CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE is selected by arm, arm64, powerpc and x86

So maybe for 5.10 we take the powerpc fix ?

Christophe