[RFC PATCH 2/3] powerpc/lib: Initialize a temporary mm for code patching

[Christopher M Riedl]

> On March 24, 2020 11:10 AM Christophe Leroy <christophe.leroy at 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 <cmr at 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