From: Daniel Axtens <dja@axtens.net>
To: "Christopher M. Riedl" <cmr@informatik.wtf>,
linuxppc-dev@lists.ozlabs.org
Cc: kernel-hardening@lists.openwall.com
Subject: Re: [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching
Date: Thu, 06 Aug 2020 13:24:31 +1000 [thread overview]
Message-ID: <87lfiso3rk.fsf@dja-thinkpad.axtens.net> (raw)
In-Reply-To: <20200709040316.12789-3-cmr@informatik.wtf>
"Christopher M. Riedl" <cmr@informatik.wtf> 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 <cmr@informatik.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 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
next prev parent reply other threads:[~2020-08-06 3:26 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-09 4:03 [PATCH 0/5] Use per-CPU temporary mappings for patching Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 1/5] powerpc/mm: Introduce temporary mm Christopher M. Riedl
2020-08-06 1:27 ` Daniel Axtens
2020-08-17 5:16 ` Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching Christopher M. Riedl
2020-07-17 8:57 ` kernel test robot
2020-08-06 3:24 ` Daniel Axtens [this message]
2020-08-17 2:21 ` Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 3/5] powerpc/lib: Use " Christopher M. Riedl
2020-07-09 7:02 ` Christophe Leroy
2020-07-14 19:43 ` Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 4/5] powerpc/lib: Add LKDTM accessor for patching addr Christopher M. Riedl
2020-07-09 4:03 ` [PATCH v2 5/5] powerpc: Add LKDTM test to hijack a patch mapping Christopher M. Riedl
2020-07-14 21:24 ` Kees Cook
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87lfiso3rk.fsf@dja-thinkpad.axtens.net \
--to=dja@axtens.net \
--cc=cmr@informatik.wtf \
--cc=kernel-hardening@lists.openwall.com \
--cc=linuxppc-dev@lists.ozlabs.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).