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Message-ID: <ce7d8643-d7bc-5d1a-6098-2352550e3793@c-s.fr>
Date: Wed, 29 Apr 2020 07:52:47 +0200
From: Christophe Leroy <christophe.leroy@....fr>
To: "Christopher M. Riedl" <cmr@...ormatik.wtf>,
linuxppc-dev@...ts.ozlabs.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH v2 3/5] powerpc/lib: Use a temporary mm for code
patching
Le 29/04/2020 à 04:05, Christopher M. Riedl a écrit :
> Currently, code patching a STRICT_KERNEL_RWX exposes the temporary
> mappings to other CPUs. These mappings should be kept local to the CPU
> doing the patching. Use the pre-initialized temporary mm and patching
> address for this purpose. Also add a check after patching to ensure the
> patch succeeded.
>
> Use the KUAP functions on non-BOOKS3_64 platforms since the temporary
> mapping for patching uses a userspace address (to keep the mapping
> local). On BOOKS3_64 platforms hash does not implement KUAP and on radix
> the use of PAGE_KERNEL sets EAA[0] for the PTE which means the AMR
> (KUAP) protection is ignored (see PowerISA v3.0b, Fig, 35).
>
> Based on x86 implementation:
>
> commit b3fd8e83ada0
> ("x86/alternatives: Use temporary mm for text poking")
>
> Signed-off-by: Christopher M. Riedl <cmr@...ormatik.wtf>
> ---
> arch/powerpc/lib/code-patching.c | 149 ++++++++++++-------------------
> 1 file changed, 55 insertions(+), 94 deletions(-)
>
> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> index 259c19480a85..26f06cdb5d7e 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -19,6 +19,7 @@
> #include <asm/page.h>
> #include <asm/code-patching.h>
> #include <asm/setup.h>
> +#include <asm/mmu_context.h>
>
> static int __patch_instruction(unsigned int *exec_addr, unsigned int instr,
> unsigned int *patch_addr)
> @@ -72,101 +73,58 @@ void __init poking_init(void)
> pte_unmap_unlock(ptep, ptl);
> }
>
> -static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> -
> -static int text_area_cpu_up(unsigned int cpu)
> -{
> - struct vm_struct *area;
> -
> - area = get_vm_area(PAGE_SIZE, VM_ALLOC);
> - if (!area) {
> - WARN_ONCE(1, "Failed to create text area for cpu %d\n",
> - cpu);
> - return -1;
> - }
> - this_cpu_write(text_poke_area, area);
> -
> - return 0;
> -}
> -
> -static int text_area_cpu_down(unsigned int cpu)
> -{
> - free_vm_area(this_cpu_read(text_poke_area));
> - return 0;
> -}
> -
> -/*
> - * Run as a late init call. This allows all the boot time patching to be done
> - * simply by patching the code, and then we're called here prior to
> - * mark_rodata_ro(), which happens after all init calls are run. Although
> - * BUG_ON() is rude, in this case it should only happen if ENOMEM, and we judge
> - * it as being preferable to a kernel that will crash later when someone tries
> - * to use patch_instruction().
> - */
> -static int __init setup_text_poke_area(void)
> -{
> - BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
> - "powerpc/text_poke:online", text_area_cpu_up,
> - text_area_cpu_down));
> -
> - return 0;
> -}
> -late_initcall(setup_text_poke_area);
> +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_area(void *addr, unsigned long text_poke_addr)
> +static int map_patch(const void *addr, struct patch_mapping *patch_mapping)
> {
> - unsigned long pfn;
> - int err;
> + struct page *page;
> + pte_t pte;
> + pgprot_t pgprot;
>
> if (is_vmalloc_addr(addr))
> - pfn = vmalloc_to_pfn(addr);
> + page = vmalloc_to_page(addr);
> else
> - pfn = __pa_symbol(addr) >> PAGE_SHIFT;
> + page = virt_to_page(addr);
>
> - err = map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL);
> + if (radix_enabled())
> + pgprot = PAGE_KERNEL;
> + else
> + pgprot = PAGE_SHARED;
>
> - pr_devel("Mapped addr %lx with pfn %lx:%d\n", text_poke_addr, pfn, err);
> - if (err)
> + 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;
> + }
> +
> + pte = mk_pte(page, pgprot);
> + if (!IS_ENABLED(CONFIG_PPC_BOOK3S_64))
> + pte = pte_mkdirty(pte);
Why only when CONFIG_PPC_BOOK3S_64 is not set ?
PAGE_KERNEL should already be dirty, so making it dirty all the time
shouldn't hurt.
> + set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte);
> +
> + init_temp_mm(&patch_mapping->temp_mm, patching_mm);
> + use_temporary_mm(&patch_mapping->temp_mm);
>
> return 0;
> }
>
> -static inline int unmap_patch_area(unsigned long addr)
> +static void unmap_patch(struct patch_mapping *patch_mapping)
> {
> - pte_t *ptep;
> - pmd_t *pmdp;
> - pud_t *pudp;
> - pgd_t *pgdp;
> -
> - pgdp = pgd_offset_k(addr);
> - if (unlikely(!pgdp))
> - return -EINVAL;
> -
> - pudp = pud_offset(pgdp, addr);
> - if (unlikely(!pudp))
> - return -EINVAL;
> -
> - pmdp = pmd_offset(pudp, addr);
> - if (unlikely(!pmdp))
> - return -EINVAL;
> -
> - ptep = pte_offset_kernel(pmdp, addr);
> - if (unlikely(!ptep))
> - return -EINVAL;
> + /* In hash, pte_clear flushes the tlb */
> + pte_clear(patching_mm, patching_addr, patch_mapping->ptep);
> + unuse_temporary_mm(&patch_mapping->temp_mm);
>
> - pr_devel("clearing mm %p, pte %p, addr %lx\n", &init_mm, ptep, addr);
> -
> - /*
> - * In hash, pte_clear flushes the tlb, in radix, we have to
> - */
> - pte_clear(&init_mm, addr, ptep);
> - flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> -
> - return 0;
> + /* In radix, we have to explicitly flush the tlb (no-op in hash) */
> + local_flush_tlb_mm(patching_mm);
> + pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl);
> }
>
> static int do_patch_instruction(unsigned int *addr, unsigned int instr)
> @@ -174,33 +132,36 @@ static int do_patch_instruction(unsigned int *addr, unsigned int instr)
> int err;
> unsigned int *patch_addr = NULL;
> unsigned long flags;
> - unsigned long text_poke_addr;
> - unsigned long kaddr = (unsigned long)addr;
> + 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
> + * The patching_mm is initialized before calling mark_rodata_ro. Prior
> + * to this, patch_instruction is called when we don't have (and don't
> + * need) the patching_mm so just do plain old patching.
> */
> - if (!this_cpu_read(text_poke_area))
> + if (!patching_mm)
> return raw_patch_instruction(addr, instr);
>
> local_irq_save(flags);
>
> - text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr;
> - if (map_patch_area(addr, text_poke_addr)) {
> - err = -1;
> + err = map_patch(addr, &patch_mapping);
> + if (err)
> goto out;
> - }
>
> - patch_addr = (unsigned int *)(text_poke_addr) +
> - ((kaddr & ~PAGE_MASK) / sizeof(unsigned int));
> + patch_addr = (unsigned int *)(patching_addr | offset_in_page(addr));
>
> - __patch_instruction(addr, instr, patch_addr);
> + if (!radix_enabled())
> + allow_write_to_user(patch_addr, sizeof(instr));
> + err = __patch_instruction(addr, instr, patch_addr);
> + if (!radix_enabled())
> + prevent_write_to_user(patch_addr, sizeof(instr));
>
> - err = unmap_patch_area(text_poke_addr);
> - if (err)
> - pr_warn("failed to unmap %lx\n", text_poke_addr);
> + unmap_patch(&patch_mapping);
> + /*
> + * Something is wrong if what we just wrote doesn't match what we
> + * think we just wrote.
> + */
> + WARN_ON(*addr != instr);
>
> out:
> local_irq_restore(flags);
>
Christophe
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