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Date: Wed, 10 Jun 2020 22:29:19 -0500
From: "Christopher M. Riedl" <cmr@...ormatik.wtf>
To: "Christophe Leroy" <christophe.leroy@...roup.eu>,
<linuxppc-dev@...ts.ozlabs.org>, <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 2/5] powerpc/lib: Initialize a temporary mm for code
patching
On Wed Jun 3, 2020 at 9:01 AM, Christophe Leroy wrote:
>
>
>
>
> 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 ?
>
We pre-allocate the PTE here since later the allocation may fail
(GFP_KERNEL) badly when interrupts are disabled during patching.
>
> Christophe
>
>
> > +}
> > +
> > static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> >
> > static int text_area_cpu_up(unsigned int cpu)
> >
>
>
>
>
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