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Riedl" To: linuxppc-dev@lists.ozlabs.org Cc: linux-hardening@vger.kernel.org Subject: [PATCH v6 4/4] powerpc/64s: Initialize and use a temporary mm for patching on Radix Date: Fri, 10 Sep 2021 21:29:04 -0500 Message-Id: <20210911022904.30962-5-cmr@bluescreens.de> In-Reply-To: <20210911022904.30962-1-cmr@bluescreens.de> References: <20210911022904.30962-1-cmr@bluescreens.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 1293F26B Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org 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 --- 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 #include #include +#include #include #include @@ -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 */ + 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); + 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; + + /* + * 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(); + 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); + } 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