Re: [PATCH v5 6/8] powerpc: Rework and improve STRICT_KERNEL_RWX patching

["Christopher M. Riedl" <cmr@linux.ibm.com>]

On Thu Aug 5, 2021 at 4:34 AM CDT, Christophe Leroy wrote:
>
>
> Le 13/07/2021 à 07:31, Christopher M. Riedl a écrit :
> > Rework code-patching with STRICT_KERNEL_RWX to prepare for the next
> > patch which uses a temporary mm for patching under the Book3s64 Radix
> > MMU. Make improvements by adding a WARN_ON when the patchsite doesn't
> > match after patching and return the error from __patch_instruction()
> > properly.
> > 
> > Signed-off-by: Christopher M. Riedl <cmr@linux.ibm.com>
> > 
> > ---
> > 
> > v5:  * New to series.
> > ---
> >   arch/powerpc/lib/code-patching.c | 51 +++++++++++++++++---------------
> >   1 file changed, 27 insertions(+), 24 deletions(-)
> > 
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> > index 3122d8e4cc013..9f2eba9b70ee4 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -102,11 +102,12 @@ static inline void unuse_temporary_mm(struct temp_mm *temp_mm)
> >   }
> >   
> >   static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> > +static DEFINE_PER_CPU(unsigned long, cpu_patching_addr);
> >   
> >   #if IS_BUILTIN(CONFIG_LKDTM)
> >   unsigned long read_cpu_patching_addr(unsigned int cpu)
> >   {
> > -	return (unsigned long)(per_cpu(text_poke_area, cpu))->addr;
> > +	return per_cpu(cpu_patching_addr, cpu);
> >   }
> >   #endif
> >   
> > @@ -121,6 +122,7 @@ static int text_area_cpu_up(unsigned int cpu)
> >   		return -1;
> >   	}
> >   	this_cpu_write(text_poke_area, area);
> > +	this_cpu_write(cpu_patching_addr, (unsigned long)area->addr);
> >   
> >   	return 0;
> >   }
> > @@ -146,7 +148,7 @@ void __init poking_init(void)
> >   /*
> >    * 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_area(void *addr)
> >   {
> >   	unsigned long pfn;
> >   	int err;
> > @@ -156,17 +158,20 @@ static int map_patch_area(void *addr, unsigned long text_poke_addr)
> >   	else
> >   		pfn = __pa_symbol(addr) >> PAGE_SHIFT;
> >   
> > -	err = map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL);
> > +	err = map_kernel_page(__this_cpu_read(cpu_patching_addr),
> > +			      (pfn << PAGE_SHIFT), PAGE_KERNEL);
> >   
> > -	pr_devel("Mapped addr %lx with pfn %lx:%d\n", text_poke_addr, pfn, err);
> > +	pr_devel("Mapped addr %lx with pfn %lx:%d\n",
> > +		 __this_cpu_read(cpu_patching_addr), pfn, err);
> >   	if (err)
> >   		return -1;
> >   
> >   	return 0;
> >   }
> >   
> > -static inline int unmap_patch_area(unsigned long addr)
> > +static inline int unmap_patch_area(void)
> >   {
> > +	unsigned long addr = __this_cpu_read(cpu_patching_addr);
> >   	pte_t *ptep;
> >   	pmd_t *pmdp;
> >   	pud_t *pudp;
> > @@ -175,23 +180,23 @@ static inline int unmap_patch_area(unsigned long addr)
> >   
> >   	pgdp = pgd_offset_k(addr);
> >   	if (unlikely(!pgdp))
> > -		return -EINVAL;
> > +		goto out_err;
> >   
> >   	p4dp = p4d_offset(pgdp, addr);
> >   	if (unlikely(!p4dp))
> > -		return -EINVAL;
> > +		goto out_err;
> >   
> >   	pudp = pud_offset(p4dp, addr);
> >   	if (unlikely(!pudp))
> > -		return -EINVAL;
> > +		goto out_err;
> >   
> >   	pmdp = pmd_offset(pudp, addr);
> >   	if (unlikely(!pmdp))
> > -		return -EINVAL;
> > +		goto out_err;
> >   
> >   	ptep = pte_offset_kernel(pmdp, addr);
> >   	if (unlikely(!ptep))
> > -		return -EINVAL;
> > +		goto out_err;
> >   
> >   	pr_devel("clearing mm %p, pte %p, addr %lx\n", &init_mm, ptep, addr);
> >   
> > @@ -202,15 +207,17 @@ static inline int unmap_patch_area(unsigned long addr)
> >   	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> >   
> >   	return 0;
> > +
> > +out_err:
> > +	pr_warn("failed to unmap %lx\n", addr);
> > +	return -EINVAL;
>
> Can you keep that in the caller of unmap_patch_area() instead of all
> those goto stuff ?
>

Yeah I think that's fair. I'll do this in the next spin.

> >   }
> >   
> >   static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
> >   {
> > -	int err;
> > +	int err, rc = 0;
> >   	u32 *patch_addr = NULL;
> >   	unsigned long flags;
> > -	unsigned long text_poke_addr;
> > -	unsigned long kaddr = (unsigned long)addr;
> >   
> >   	/*
> >   	 * During early early boot patch_instruction is called
> > @@ -222,24 +229,20 @@ static int do_patch_instruction(u32 *addr, struct ppc_inst 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_area(addr);
> > +	if (err)
> >   		goto out;
> > -	}
> > -
> > -	patch_addr = (u32 *)(text_poke_addr + (kaddr & ~PAGE_MASK));
> >   
> > -	__patch_instruction(addr, instr, patch_addr);
> > +	patch_addr = (u32 *)(__this_cpu_read(cpu_patching_addr) | offset_in_page(addr));
> > +	rc = __patch_instruction(addr, instr, patch_addr);
> >   
> > -	err = unmap_patch_area(text_poke_addr);
> > -	if (err)
> > -		pr_warn("failed to unmap %lx\n", text_poke_addr);
> > +	err = unmap_patch_area();
> >   
> >   out:
> >   	local_irq_restore(flags);
> > +	WARN_ON(!ppc_inst_equal(ppc_inst_read(addr), instr));
>
> Why adding that WARN_ON(), what could make that happen that is worth a
> WARN_ON() ?

Failing to patch something could cause very strange issues later, so
explicitly calling out a failure when it happens is warranted IMO.

>
> Patching is quite a critical fast path, I'm not sure we want to afford
> too many checks during
> patching, we want it quick at first.

Hmm, I'd prefer to measure the impact first - if it's a huge degradation
then sure we can drop the WARN_ON()... I'll add some data with the next
spin.

>
> >   
> > -	return err;
> > +	return rc ? rc : err;
> >   }
> >   #else /* !CONFIG_STRICT_KERNEL_RWX */
> >   
> >