linux-hardening - Re: [RESEND PATCH v4 08/11] powerpc: Initialize and use a temporary mm for patching

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Date:   Mon, 21 Jun 2021 13:19:10 +1000
From:   Daniel Axtens <dja@...ens.net>
To:     "Christopher M. Riedl" <cmr@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     tglx@...utronix.de, x86@...nel.org,
        linux-hardening@...r.kernel.org, keescook@...omium.org
Subject: Re: [RESEND PATCH v4 08/11] powerpc: Initialize and use a temporary
 mm for patching

Hi Chris,

> +	/*
> +	 * Choose a randomized, page-aligned address from the range:
> +	 * [PAGE_SIZE, DEFAULT_MAP_WINDOW - PAGE_SIZE]
> +	 * The lower address bound is PAGE_SIZE to avoid the zero-page.
> +	 * The upper address bound is DEFAULT_MAP_WINDOW - PAGE_SIZE to stay
> +	 * under DEFAULT_MAP_WINDOW with the Book3s64 Hash MMU.
> +	 */
> +	patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK)
> +			% (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE));

I checked and poking_init() comes after the functions that init the RNG,
so this should be fine. The maths - while a bit fiddly to reason about -
does check out.

> +
> +	/*
> +	 * PTE allocation uses GFP_KERNEL which means we need to pre-allocate
> +	 * the PTE here. We cannot do the allocation during patching with IRQs
> +	 * disabled (ie. "atomic" context).
> +	 */
> +	ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
> +	BUG_ON(!ptep);
> +	pte_unmap_unlock(ptep, ptl);
> +}
>  
>  #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 patching_addr;
>  }
>  #endif
>  
> -static int text_area_cpu_up(unsigned int cpu)
> +struct patch_mapping {
> +	spinlock_t *ptl; /* for protecting pte table */
> +	pte_t *ptep;
> +	struct temp_mm temp_mm;
> +};
> +
> +#ifdef CONFIG_PPC_BOOK3S_64
> +
> +static inline int hash_prefault_mapping(pgprot_t pgprot)
>  {
> -	struct vm_struct *area;
> +	int err;
>  
> -	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);
> +	if (radix_enabled())
> +		return 0;
>  
> -	return 0;
> -}
> +	err = slb_allocate_user(patching_mm, patching_addr);
> +	if (err)
> +		pr_warn("map patch: failed to allocate slb entry\n");
>  

Here if slb_allocate_user() fails, you'll print a warning and then fall
through to the rest of the function. You do return err, but there's a
later call to hash_page_mm() that also sets err. Can slb_allocate_user()
fail while hash_page_mm() succeeds, and would that be a problem?

> -static int text_area_cpu_down(unsigned int cpu)
> -{
> -	free_vm_area(this_cpu_read(text_poke_area));
> -	return 0;
> +	err = hash_page_mm(patching_mm, patching_addr, pgprot_val(pgprot), 0,
> +			   HPTE_USE_KERNEL_KEY);
> +	if (err)
> +		pr_warn("map patch: failed to insert hashed page\n");
> +
> +	/* See comment in switch_slb() in mm/book3s64/slb.c */
> +	isync();
> +

The comment reads:

	/*
	 * Synchronize slbmte preloads with possible subsequent user memory
	 * address accesses by the kernel (user mode won't happen until
	 * rfid, which is safe).
	 */
         isync();

I have to say having read the description of isync I'm not 100% sure why
that's enough (don't we also need stores to complete?) but I'm happy to
take commit 5434ae74629a ("powerpc/64s/hash: Add a SLB preload cache")
on trust here!

I think it does make sense for you to have that barrier here: you are
potentially about to start poking at the memory mapped through that SLB
entry so you should make sure you're fully synchronised.

> +	return err;
>  }
>  

> +	init_temp_mm(&patch_mapping->temp_mm, patching_mm);
> +	use_temporary_mm(&patch_mapping->temp_mm);
>  
> -	pmdp = pmd_offset(pudp, addr);
> -	if (unlikely(!pmdp))
> -		return -EINVAL;
> +	/*
> +	 * On Book3s64 with the Hash MMU we have to manually insert the SLB
> +	 * entry and HPTE to prevent taking faults on the patching_addr later.
> +	 */
> +	return(hash_prefault_mapping(pgprot));

hmm, `return hash_prefault_mapping(pgprot);` or
`return (hash_prefault_mapping((pgprot));` maybe?

Kind regards,
Daniel