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Message-ID: <4ffced42-ee3a-841f-2d3f-34daec11b05b@csgroup.eu>
Date: Wed, 3 Jun 2020 09:01:08 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Christopher M. Riedl" <cmr@...ormatik.wtf>,
linuxppc-dev@...ts.ozlabs.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 2/5] powerpc/lib: Initialize a temporary mm for code
patching
Le 03/06/2020 à 07:19, Christopher M. Riedl a écrit :
> 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 <cmr@...ormatik.wtf>
> ---
> 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 5ecf0d635a8d..599114f63b44 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/pgtable.h>
> #include <asm/tlbflush.h>
> @@ -45,6 +47,37 @@ 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);
> +
> + ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
> + BUG_ON(!ptep);
> + pte_unmap_unlock(ptep, ptl);
Is this needed ? What's the point in getting the pte to unmap it
immediatly without doing anything with it ?
Christophe
> +}
> +
> static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
>
> static int text_area_cpu_up(unsigned int cpu)
>
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