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Date:   Sat, 11 Sep 2021 19:14:28 +1000
From:   Jordan Niethe <jniethe5@...il.com>
To:     "Christopher M. Riedl" <cmr@...escreens.de>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v6 4/4] powerpc/64s: Initialize and use a temporary mm for
 patching on Radix

On Sat, Sep 11, 2021 at 12:39 PM Christopher M. Riedl
<cmr@...escreens.de> wrote:
>
> When code patching a STRICT_KERNEL_RWX kernel the page containing the
> address to be patched is temporarily mapped as writeable. 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 patching. The mapping is exposed to CPUs
> other than the patching CPU - this is undesirable from a hardening
> perspective. Use a temporary mm instead which keeps the mapping local to
> the CPU doing the patching.
>
> 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
> space. The patching address is randomized between PAGE_SIZE and
> DEFAULT_MAP_WINDOW-PAGE_SIZE.
>
> Bits of entropy with 64K page size on BOOK3S_64:
>
>         bits of entropy = log2(DEFAULT_MAP_WINDOW_USER64 / PAGE_SIZE)
>
>         PAGE_SIZE=64K, DEFAULT_MAP_WINDOW_USER64=128TB
>         bits of entropy = log2(128TB / 64K)
>         bits of entropy = 31
>
> The upper limit is DEFAULT_MAP_WINDOW due to how the Book3s64 Hash MMU
> operates - by default the space above DEFAULT_MAP_WINDOW is not
> available. Currently the Hash MMU does not use a temporary mm so
> technically this upper limit isn't necessary; however, a larger
> randomization range does not further "harden" this overall approach and
> future work may introduce patching with a temporary mm on Hash as well.
>
> Randomization occurs only once during initialization at boot for each
> possible CPU in the system.
>
> Introduce two new functions, map_patch_mm() and unmap_patch_mm(), to
> respectively create and remove the temporary mapping with write
> permissions at patching_addr. Map the page with PAGE_KERNEL to set
> EAA[0] for the PTE which ignores the AMR (so no need to unlock/lock
> KUAP) according to PowerISA v3.0b Figure 35 on Radix.
>
> Based on x86 implementation:
>
> commit 4fc19708b165
> ("x86/alternatives: Initialize temporary mm for patching")
>
> and:
>
> commit b3fd8e83ada0
> ("x86/alternatives: Use temporary mm for text poking")
>
> Signed-off-by: Christopher M. Riedl <cmr@...escreens.de>
>
> ---
>
> v6:  * Small clean-ups (naming, formatting, style, etc).
>      * Call stop_using_temporary_mm() before pte_unmap_unlock() after
>        patching.
>      * Replace BUG_ON()s in poking_init() w/ WARN_ON()s.
>
> v5:  * Only support Book3s64 Radix MMU for now.
>      * Use a per-cpu datastructure to hold the patching_addr and
>        patching_mm to avoid the need for a synchronization lock/mutex.
>
> v4:  * In the previous series this was two separate patches: one to init
>        the temporary mm in poking_init() (unused in powerpc at the time)
>        and the other to use it for patching (which removed all the
>        per-cpu vmalloc code). Now that we use poking_init() in the
>        existing per-cpu vmalloc approach, that separation doesn't work
>        as nicely anymore so I just merged the two patches into one.
>      * Preload the SLB entry and hash the page for the patching_addr
>        when using Hash on book3s64 to avoid taking an SLB and Hash fault
>        during patching. The previous implementation was a hack which
>        changed current->mm to allow the SLB and Hash fault handlers to
>        work with the temporary mm since both of those code-paths always
>        assume mm == current->mm.
>      * Also (hmm - seeing a trend here) with the book3s64 Hash MMU we
>        have to manage the mm->context.active_cpus counter and mm cpumask
>        since they determine (via mm_is_thread_local()) if the TLB flush
>        in pte_clear() is local or not - it should always be local when
>        we're using the temporary mm. On book3s64's Radix MMU we can
>        just call local_flush_tlb_mm().
>      * Use HPTE_USE_KERNEL_KEY on Hash to avoid costly lock/unlock of
>        KUAP.
> ---
>  arch/powerpc/lib/code-patching.c | 119 +++++++++++++++++++++++++++++--
>  1 file changed, 112 insertions(+), 7 deletions(-)
>
> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> index e802e42c2789..af8e2a02a9dd 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -11,6 +11,7 @@
>  #include <linux/cpuhotplug.h>
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
> +#include <linux/random.h>
>
>  #include <asm/tlbflush.h>
>  #include <asm/page.h>
> @@ -103,6 +104,7 @@ static inline void stop_using_temporary_mm(struct temp_mm *temp_mm)
>
>  static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
>  static DEFINE_PER_CPU(unsigned long, cpu_patching_addr);
> +static DEFINE_PER_CPU(struct mm_struct *, cpu_patching_mm);
>
>  static int text_area_cpu_up(unsigned int cpu)
>  {
> @@ -126,8 +128,48 @@ static int text_area_cpu_down(unsigned int cpu)
>         return 0;
>  }
>
> +static __always_inline void __poking_init_temp_mm(void)
> +{
> +       int cpu;
> +       spinlock_t *ptl; /* for protecting pte table */

ptl is just used so we don't have to open code allocating a pte in
patching_mm isn't it?

> +       pte_t *ptep;
> +       struct mm_struct *patching_mm;
> +       unsigned long patching_addr;
> +
> +       for_each_possible_cpu(cpu) {
> +               patching_mm = copy_init_mm();
> +               WARN_ON(!patching_mm);

Would it be okay to just let the mmu handle null pointer dereferences?

> +               per_cpu(cpu_patching_mm, cpu) = patching_mm;
> +
> +               /*
> +                * 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));
> +               per_cpu(cpu_patching_addr, cpu) = patching_addr;

On x86 the randomization depends on CONFIG_RANDOMIZE_BASE. Should it
be controllable here too?

> +
> +               /*
> +                * PTE allocation uses GFP_KERNEL which means we need to
> +                * pre-allocate the PTE here because we cannot do the
> +                * allocation during patching when IRQs are disabled.
> +                */
> +               ptep = get_locked_pte(patching_mm, patching_addr, &ptl);
> +               WARN_ON(!ptep);
> +               pte_unmap_unlock(ptep, ptl);
> +       }
> +}
> +
>  void __init poking_init(void)
>  {
> +       if (radix_enabled()) {
> +               __poking_init_temp_mm();

Should this also be done with cpuhp_setup_state()?

> +               return;
> +       }
> +
>         WARN_ON(cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
>                 "powerpc/text_poke:online", text_area_cpu_up,
>                 text_area_cpu_down) < 0);
> @@ -197,30 +239,93 @@ static inline int unmap_patch_area(void)
>         return 0;
>  }
>
> +struct patch_mapping {
> +       spinlock_t *ptl; /* for protecting pte table */
> +       pte_t *ptep;
> +       struct temp_mm temp_mm;
> +};
> +
> +/*
> + * This can be called for kernel text or a module.
> + */
> +static int map_patch_mm(const void *addr, struct patch_mapping *patch_mapping)
> +{
> +       struct page *page;
> +       struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm);
> +       unsigned long patching_addr = __this_cpu_read(cpu_patching_addr);
> +
> +       if (is_vmalloc_or_module_addr(addr))
> +               page = vmalloc_to_page(addr);
> +       else
> +               page = virt_to_page(addr);
> +
> +       patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr,
> +                                            &patch_mapping->ptl);
> +       if (unlikely(!patch_mapping->ptep)) {
> +               pr_warn("map patch: failed to allocate pte for patching\n");
> +               return -1;
> +       }
> +
> +       set_pte_at(patching_mm, patching_addr, patch_mapping->ptep,
> +                  pte_mkdirty(mk_pte(page, PAGE_KERNEL)));
> +
> +       init_temp_mm(&patch_mapping->temp_mm, patching_mm);
> +       start_using_temporary_mm(&patch_mapping->temp_mm);
> +
> +       return 0;
> +}
> +
> +static int unmap_patch_mm(struct patch_mapping *patch_mapping)
> +{
> +       struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm);
> +       unsigned long patching_addr = __this_cpu_read(cpu_patching_addr);
> +
> +       pte_clear(patching_mm, patching_addr, patch_mapping->ptep);
> +
> +       local_flush_tlb_mm(patching_mm);
> +       stop_using_temporary_mm(&patch_mapping->temp_mm);
> +
> +       pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl);
> +
> +       return 0;
> +}
> +
>  static int do_patch_instruction(u32 *addr, struct ppc_inst instr)
>  {
>         int err, rc = 0;
>         u32 *patch_addr = NULL;
>         unsigned long flags;
> +       struct patch_mapping patch_mapping;
>
>         /*
> -        * During early early boot patch_instruction is called
> -        * when text_poke_area is not ready, but we still need
> -        * to allow patching. We just do the plain old patching
> +        * During early early boot patch_instruction is called when the
> +        * patching_mm/text_poke_area is not ready, but we still need to allow
> +        * patching. We just do the plain old patching.
>          */
> -       if (!this_cpu_read(text_poke_area))
> -               return raw_patch_instruction(addr, instr);
> +       if (radix_enabled()) {
> +               if (!this_cpu_read(cpu_patching_mm))
> +                       return raw_patch_instruction(addr, instr);
> +       } else {
> +               if (!this_cpu_read(text_poke_area))
> +                       return raw_patch_instruction(addr, instr);
> +       }

Would testing cpu_patching_addr handler both of these cases?

Then I think it might be clearer to do something like this:
if (radix_enabled()) {
      return patch_instruction_mm(addr, instr);
}

patch_instruction_mm() would combine map_patch_mm(), then patching and
unmap_patch_mm() into one function.

IMO, a bit of code duplication would be cleaner than checking multiple
times for radix_enabled() and having struct patch_mapping especially
for maintaining state.

>
>         local_irq_save(flags);
>
> -       err = map_patch_area(addr);
> +       if (radix_enabled())
> +               err = map_patch_mm(addr, &patch_mapping);
> +       else
> +               err = map_patch_area(addr);
>         if (err)
>                 goto out;
>
>         patch_addr = (u32 *)(__this_cpu_read(cpu_patching_addr) | offset_in_page(addr));
>         rc = __patch_instruction(addr, instr, patch_addr);
>
> -       err = unmap_patch_area();
> +       if (radix_enabled())
> +               err = unmap_patch_mm(&patch_mapping);
> +       else
> +               err = unmap_patch_area();
>
>  out:
>         local_irq_restore(flags);
> --
> 2.32.0
>
Thanks,
Jordan

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