Subject: Re: [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching

From: Daniel Axtens <[email-masked]>
Date: Thu, 06 Aug 2020 13:24:31 +1000

"Christopher M. Riedl" <[email-masked]> writes:

> 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 <[email-masked]>
> ---
>  arch/powerpc/lib/code-patching.c | 33 ++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
>
> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> index 0a051dfeb177..8ae1a9e5fe6e 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/tlbflush.h>
>  #include <asm/page.h>
> @@ -44,6 +46,37 @@ int raw_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
>  }
>  
>  #ifdef CONFIG_STRICT_KERNEL_RWX
> +
> +static struct mm_struct *patching_mm __ro_after_init;
> +static unsigned long patching_addr __ro_after_init;
> +
> +void __init poking_init(void)
> +{
> +	spinlock_t *ptl; /* for protecting pte table */
> +	pte_t *ptep;
> +
> +	/*
> +	 * Some parts of the kernel (static keys for example) depend on
> +	 * successful code patching. Code patching under STRICT_KERNEL_RWX
> +	 * requires this setup - otherwise we cannot patch at all. We use
> +	 * BUG_ON() here and later since an early failure is preferred to
> +	 * buggy behavior and/or strange crashes later.
> +	 */
> +	patching_mm = copy_init_mm();
> +	BUG_ON(!patching_mm);
> +
> +	/*
> +	 * 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);

It took me a while to understand this calculation. I see that it's
calculating a base address for a page in which to do patching. It does
the following:

 - get a random long

 - mask with PAGE_MASK so as to get a page aligned value

 - make sure that the base address is at least one PAGE_SIZE below
   DEFAULT_MAP_WINDOW so we have a clear page between the base and
   DEFAULT_MAP_WINDOW.

On 64-bit Book3S with 64K pages, that works out to be

PAGE_SIZE = 0x0000 0000 0001 0000
PAGE_MASK = 0xFFFF FFFF FFFF 0000

DEFAULT_MAP_WINDOW = DEFAULT_MAP_WINDOW_USER64 = TASK_SIZE_128TB
                   = 0x0000_8000_0000_0000

DEFAULT_MAP_WINDOW - PAGE_SIZE = 0x0000 7FFF FFFF 0000

It took a while (and a conversation with my wife who studied pure
maths!) but I am convinced that the modulo preserves the page-alignement
of the patching address.

One thing I did realise is that patching_addr can be zero at the end of
this process. That seems dubious and slightly error-prone to me - is
the patching process robust to that or should we exclude it?

Anyway, if I have the maths right, that there are 0x7fffffff or ~2
billion possible locations for the patching page, which is just shy of
31 bits of entropy.

I think this compares pretty favourably to most (K)ASLR implementations?

What's the range if built with 4k pages?

Kind regards,
Daniel

> +
> +	ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
> +	BUG_ON(!ptep);
> +	pte_unmap_unlock(ptep, ptl);
> +}
> +
>  static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
>  
>  static int text_area_cpu_up(unsigned int cpu)
> -- 
> 2.27.0