Subject: Re: [PATCH v2 3/5] powerpc/lib: Use a temporary mm for code patching

From: "Christopher M. Riedl" <[email-masked]>
Date: Tue, 14 Jul 2020 14:43:50 -0500

On Thu Jul 9, 2020 at 4:02 AM CDT, Christophe Leroy wrote:
>
>
> Le 09/07/2020 à 06:03, Christopher M. Riedl a écrit :
> > Currently, code patching a STRICT_KERNEL_RWX exposes the temporary
> > mappings to other CPUs. These mappings should be kept local to the CPU
> > doing the patching. Use the pre-initialized temporary mm and patching
> > address for this purpose. Also add a check after patching to ensure the
> > patch succeeded.
>
> While trying the LKDTM test, I realised that this is useless for non
> SMP.
> Is it worth applying that change to non SMP systems ?
>
> Christophe
>

Hmm, I would say it's probably preferable to maintain a single
implementation of code-patching under STRICT_KERNEL_RWX instead of
two versions for SMP and non-SMP.

> > 
> > Use the KUAP functions on non-BOOKS3_64 platforms since the temporary
> > mapping for patching uses a userspace address (to keep the mapping
> > local). On BOOKS3_64 platforms hash does not implement KUAP and on radix
> > the use of PAGE_KERNEL sets EAA[0] for the PTE which means the AMR
> > (KUAP) protection is ignored (see PowerISA v3.0b, Fig, 35).
> > 
> > Based on x86 implementation:
> > 
> > commit b3fd8e83ada0
> > ("x86/alternatives: Use temporary mm for text poking")
> > 
> > Signed-off-by: Christopher M. Riedl <[email-masked]>
> > ---
> >   arch/powerpc/lib/code-patching.c | 152 +++++++++++--------------------
> >   1 file changed, 54 insertions(+), 98 deletions(-)
> > 
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> > index 8ae1a9e5fe6e..80fe3864f377 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -19,6 +19,7 @@
> >   #include <asm/code-patching.h>
> >   #include <asm/setup.h>
> >   #include <asm/inst.h>
> > +#include <asm/mmu_context.h>
> >   
> >   static int __patch_instruction(struct ppc_inst *exec_addr, struct ppc_inst instr,
> >   			       struct ppc_inst *patch_addr)
> > @@ -77,106 +78,57 @@ void __init poking_init(void)
> >   	pte_unmap_unlock(ptep, ptl);
> >   }
> >   
> > -static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> > -
> > -static int text_area_cpu_up(unsigned int cpu)
> > -{
> > -	struct vm_struct *area;
> > -
> > -	area = get_vm_area(PAGE_SIZE, VM_ALLOC);
> > -	if (!area) {
> > -		WARN_ONCE(1, "Failed to create text area for cpu %d\n",
> > -			cpu);
> > -		return -1;
> > -	}
> > -	this_cpu_write(text_poke_area, area);
> > -
> > -	return 0;
> > -}
> > -
> > -static int text_area_cpu_down(unsigned int cpu)
> > -{
> > -	free_vm_area(this_cpu_read(text_poke_area));
> > -	return 0;
> > -}
> > -
> > -/*
> > - * 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);
> > +struct patch_mapping {
> > +	spinlock_t *ptl; /* for protecting pte table */
> > +	pte_t *ptep;
> > +	struct temp_mm temp_mm;
> > +};
> >   
> >   /*
> >    * This can be called for kernel text or a module.
> >    */
> > -static int map_patch_area(void *addr, unsigned long text_poke_addr)
> > +static int map_patch(const void *addr, struct patch_mapping *patch_mapping)
> >   {
> > -	unsigned long pfn;
> > -	int err;
> > +	struct page *page;
> > +	pte_t pte;
> > +	pgprot_t pgprot;
> >   
> >   	if (is_vmalloc_addr(addr))
> > -		pfn = vmalloc_to_pfn(addr);
> > +		page = vmalloc_to_page(addr);
> >   	else
> > -		pfn = __pa_symbol(addr) >> PAGE_SHIFT;
> > +		page = virt_to_page(addr);
> >   
> > -	err = map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL);
> > +	if (radix_enabled())
> > +		pgprot = PAGE_KERNEL;
> > +	else
> > +		pgprot = PAGE_SHARED;
> >   
> > -	pr_devel("Mapped addr %lx with pfn %lx:%d\n", text_poke_addr, pfn, err);
> > -	if (err)
> > +	patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr,
> > +					     &patch_mapping->ptl);
> > +	if (unlikely(!patch_mapping->ptep)) {
> > +		pr_warn("map patch: failed to allocate pte for patching\n");
> >   		return -1;
> > +	}
> > +
> > +	pte = mk_pte(page, pgprot);
> > +	pte = pte_mkdirty(pte);
> > +	set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte);
> > +
> > +	init_temp_mm(&patch_mapping->temp_mm, patching_mm);
> > +	use_temporary_mm(&patch_mapping->temp_mm);
> >   
> >   	return 0;
> >   }
> >   
> > -static inline int unmap_patch_area(unsigned long addr)
> > +static void unmap_patch(struct patch_mapping *patch_mapping)
> >   {
> > -	pte_t *ptep;
> > -	pmd_t *pmdp;
> > -	pud_t *pudp;
> > -	p4d_t *p4dp;
> > -	pgd_t *pgdp;
> > -
> > -	pgdp = pgd_offset_k(addr);
> > -	if (unlikely(!pgdp))
> > -		return -EINVAL;
> > -
> > -	p4dp = p4d_offset(pgdp, addr);
> > -	if (unlikely(!p4dp))
> > -		return -EINVAL;
> > -
> > -	pudp = pud_offset(p4dp, addr);
> > -	if (unlikely(!pudp))
> > -		return -EINVAL;
> > -
> > -	pmdp = pmd_offset(pudp, addr);
> > -	if (unlikely(!pmdp))
> > -		return -EINVAL;
> > -
> > -	ptep = pte_offset_kernel(pmdp, addr);
> > -	if (unlikely(!ptep))
> > -		return -EINVAL;
> > +	/* In hash, pte_clear flushes the tlb */
> > +	pte_clear(patching_mm, patching_addr, patch_mapping->ptep);
> > +	unuse_temporary_mm(&patch_mapping->temp_mm);
> >   
> > -	pr_devel("clearing mm %p, pte %p, addr %lx\n", &init_mm, ptep, addr);
> > -
> > -	/*
> > -	 * In hash, pte_clear flushes the tlb, in radix, we have to
> > -	 */
> > -	pte_clear(&init_mm, addr, ptep);
> > -	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > -
> > -	return 0;
> > +	/* In radix, we have to explicitly flush the tlb (no-op in hash) */
> > +	local_flush_tlb_mm(patching_mm);
> > +	pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl);
> >   }
> >   
> >   static int do_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
> > @@ -184,32 +136,36 @@ static int do_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
> >   	int err;
> >   	struct ppc_inst *patch_addr = NULL;
> >   	unsigned long flags;
> > -	unsigned long text_poke_addr;
> > -	unsigned long kaddr = (unsigned long)addr;
> > +	struct patch_mapping patch_mapping;
> >   
> >   	/*
> > -	 * During early early boot patch_instruction is called
> > -	 * when text_poke_area is not ready, but we still need
> > -	 * to allow patching. We just do the plain old patching
> > +	 * The patching_mm is initialized before calling mark_rodata_ro. Prior
> > +	 * to this, patch_instruction is called when we don't have (and don't
> > +	 * need) the patching_mm so just do plain old patching.
> >   	 */
> > -	if (!this_cpu_read(text_poke_area))
> > +	if (!patching_mm)
> >   		return raw_patch_instruction(addr, instr);
> >   
> >   	local_irq_save(flags);
> >   
> > -	text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr;
> > -	if (map_patch_area(addr, text_poke_addr)) {
> > -		err = -1;
> > +	err = map_patch(addr, &patch_mapping);
> > +	if (err)
> >   		goto out;
> > -	}
> >   
> > -	patch_addr = (struct ppc_inst *)(text_poke_addr + (kaddr & ~PAGE_MASK));
> > +	patch_addr = (struct ppc_inst *)(patching_addr | offset_in_page(addr));
> >   
> > -	__patch_instruction(addr, instr, patch_addr);
> > +	if (!radix_enabled())
> > +		allow_write_to_user(patch_addr, ppc_inst_len(instr));
> > +	err = __patch_instruction(addr, instr, patch_addr);
> > +	if (!radix_enabled())
> > +		prevent_write_to_user(patch_addr, ppc_inst_len(instr));
> >   
> > -	err = unmap_patch_area(text_poke_addr);
> > -	if (err)
> > -		pr_warn("failed to unmap %lx\n", text_poke_addr);
> > +	unmap_patch(&patch_mapping);
> > +	/*
> > +	 * Something is wrong if what we just wrote doesn't match what we
> > +	 * think we just wrote.
> > +	 */
> > +	WARN_ON(!ppc_inst_equal(ppc_inst_read(addr), instr));
> >   
> >   out:
> >   	local_irq_restore(flags);
> >