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Date:   Thu, 5 Aug 2021 11:34:25 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Christopher M. Riedl" <cmr@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     keescook@...omium.org, peterz@...radead.org, x86@...nel.org,
        npiggin@...il.com, linux-hardening@...r.kernel.org,
        tglx@...utronix.de, dja@...ens.net
Subject: Re: [PATCH v5 6/8] powerpc: Rework and improve STRICT_KERNEL_RWX
 patching



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@...ux.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 ?

>   }
>   
>   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() ?

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.

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

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