> On March 24, 2020 11:10 AM Christophe Leroy <christophe.le...@c-s.fr> wrote:
>
>
> Le 23/03/2020 à 05:52, Christopher M. Riedl a écrit :
> > When code patching a STRICT_KERNEL_RWX kernel the page containing the
> > address to be patched is temporarily mapped with permissive memory
> > protections. Currently, a per-cpu vmalloc patch area is used for this
> > purpose. While the patch area is per-cpu, the temporary page mapping is
> > inserted into the kernel page tables for the duration of the patching.
> > The mapping is exposed to CPUs other than the patching CPU - this is
> > undesirable from a hardening perspective.
> >
> > Use the `poking_init` init hook to prepare a temporary mm and patching
> > address. Initialize the temporary mm by copying the init mm. Choose a
> > randomized patching address inside the temporary mm userspace address
> > portion. The next patch uses the temporary mm and patching address for
> > code patching.
> >
> > Based on x86 implementation:
> >
> > commit 4fc19708b165
> > ("x86/alternatives: Initialize temporary mm for patching")
> >
> > Signed-off-by: Christopher M. Riedl <c...@informatik.wtf>
> > ---
> > arch/powerpc/lib/code-patching.c | 26 ++++++++++++++++++++++++++
> > 1 file changed, 26 insertions(+)
> >
> > diff --git a/arch/powerpc/lib/code-patching.c
> > b/arch/powerpc/lib/code-patching.c
> > index 3345f039a876..18b88ecfc5a8 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -11,6 +11,8 @@
> > #include <linux/cpuhotplug.h>
> > #include <linux/slab.h>
> > #include <linux/uaccess.h>
> > +#include <linux/sched/task.h>
> > +#include <linux/random.h>
> >
> > #include <asm/pgtable.h>
> > #include <asm/tlbflush.h>
> > @@ -39,6 +41,30 @@ int raw_patch_instruction(unsigned int *addr, unsigned
> > int instr)
> > }
> >
> > #ifdef CONFIG_STRICT_KERNEL_RWX
> > +
> > +__ro_after_init struct mm_struct *patching_mm;
> > +__ro_after_init unsigned long patching_addr;
>
> Can we make those those static ?
>
Yes, makes sense to me.
> > +
> > +void __init poking_init(void)
> > +{
> > + spinlock_t *ptl; /* for protecting pte table */
> > + pte_t *ptep;
> > +
> > + patching_mm = copy_init_mm();
> > + BUG_ON(!patching_mm);
>
> Does it needs to be a BUG_ON() ? Can't we fail gracefully with just a
> WARN_ON ?
>
I'm not sure what failing gracefully means here? The main reason this could
fail is if there is not enough memory to allocate the patching_mm. The
previous implementation had this justification for BUG_ON():
/*
* Run as a late init call. This allows all the boot time patching to be done
* simply by patching the code, and then we're called here prior to
* mark_rodata_ro(), which happens after all init calls are run. Although
* BUG_ON() is rude, in this case it should only happen if ENOMEM, and we judge
* it as being preferable to a kernel that will crash later when someone tries
* to use patch_instruction().
*/
static int __init setup_text_poke_area(void)
{
BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
"powerpc/text_poke:online", text_area_cpu_up,
text_area_cpu_down));
return 0;
}
late_initcall(setup_text_poke_area);
I think the BUG_ON() is appropriate even if only to adhere to the previous
judgement call. I can add a similar comment explaining the reasoning if
that helps.
> > +
> > + /*
> > + * In hash we cannot go above DEFAULT_MAP_WINDOW easily.
> > + * XXX: Do we want additional bits of entropy for radix?
> > + */
> > + patching_addr = (get_random_long() & PAGE_MASK) %
> > + (DEFAULT_MAP_WINDOW - PAGE_SIZE);
> > +
> > + ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
> > + BUG_ON(!ptep);
>
> Same here, can we fail gracefully instead ?
>
Same reasoning as above.
> > + pte_unmap_unlock(ptep, ptl);
> > +}
> > +
> > static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> >
> > static int text_area_cpu_up(unsigned int cpu)
> >
>
> Christophe
