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

[Christopher M Riedl]

> On April 8, 2020 6:01 AM Christophe Leroy <christophe.leroy at c-s.fr> wrote:
> 
>  
> Le 31/03/2020 à 05:19, Christopher M Riedl a écrit :
> >> 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():
> 
> But the system can continue running just fine after this failure.
> Only the things that make use of code patching will fail (ftrace, kgdb, ...)
> 
> Checkpatch tells: "Avoid crashing the kernel - try using WARN_ON & 
> recovery code rather than BUG() or BUG_ON()"
> 
> All vital code patching has already been done previously, so I think a 
> WARN_ON() should be enough, plus returning non 0 to indicate that the 
> late_initcall failed.
> 
> 

Got it, makes sense to me. I will make these changes in the next version.
Thanks!

> > 
> > /*
> >   * 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.
> 
> Here as well, a WARN_ON() should be enough, the system will continue 
> running after that.
> 
> > 
> >>> +	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
> 
> Christophe