From ba42b524a0408b5f92bd41edaee1ea84309ab9ae Mon Sep 17 00:00:00 2001 From: York Jasper Niebuhr Date: Fri, 29 Mar 2024 15:56:05 +0100 Subject: [PATCH] mm: init_mlocked_on_free_v3 Implements the "init_mlocked_on_free" boot option. When this boot option is enabled, any mlock'ed pages are zeroed on free. If the pages are munlock'ed beforehand, no initialization takes place. This boot option is meant to combat the performance hit of "init_on_free" as reported in commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options"). With "init_mlocked_on_free=1" only relevant data is freed while everything else is left untouched by the kernel. Correspondingly, this patch introduces no performance hit for unmapping non-mlock'ed memory. The unmapping overhead for purely mlocked memory was measured to be approximately 13%. Realistically, most systems mlock only a fraction of the total memory so the real-world system overhead should be close to zero. Optimally, userspace programs clear any key material or other confidential memory before exit and munlock the according memory regions. If a program crashes, userspace key managers fail to do this job. Accordingly, no munlock operations are performed so the data is caught and zeroed by the kernel. Should the program not crash, all memory will ideally be munlocked so no overhead is caused. CONFIG_INIT_MLOCKED_ON_FREE_DEFAULT_ON can be set to enable "init_mlocked_on_free" by default. Link: https://lkml.kernel.org/r/20240329145605.149917-1-yjnworkstation@gmail.com Signed-off-by: York Jasper Niebuhr Cc: Matthew Wilcox (Oracle) Cc: York Jasper Niebuhr Cc: Kees Cook Signed-off-by: Andrew Morton --- .../admin-guide/kernel-parameters.txt | 6 +++ include/linux/mm.h | 9 +++- mm/internal.h | 1 + mm/memory.c | 6 +++ mm/mm_init.c | 43 ++++++++++++++++--- mm/page_alloc.c | 2 +- security/Kconfig.hardening | 15 +++++++ 7 files changed, 73 insertions(+), 9 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 902ecd92a29f..3ff97de349da 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2148,6 +2148,12 @@ Format: 0 | 1 Default set by CONFIG_INIT_ON_FREE_DEFAULT_ON. + init_mlocked_on_free= [MM] Fill freed userspace memory with zeroes if + it was mlock'ed and not explicitly munlock'ed + afterwards. + Format: 0 | 1 + Default set by CONFIG_INIT_MLOCKED_ON_FREE_DEFAULT_ON + init_pkru= [X86] Specify the default memory protection keys rights register contents for all processes. 0x55555554 by default (disallow access to all but pkey 0). Can diff --git a/include/linux/mm.h b/include/linux/mm.h index 2d5e492ef57f..4f4e460d7853 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3762,7 +3762,14 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_INIT_ON_FREE_DEFAULT_ON, init_on_free); static inline bool want_init_on_free(void) { return static_branch_maybe(CONFIG_INIT_ON_FREE_DEFAULT_ON, - &init_on_free); + &init_on_free); +} + +DECLARE_STATIC_KEY_MAYBE(CONFIG_INIT_MLOCKED_ON_FREE_DEFAULT_ON, init_mlocked_on_free); +static inline bool want_init_mlocked_on_free(void) +{ + return static_branch_maybe(CONFIG_INIT_MLOCKED_ON_FREE_DEFAULT_ON, + &init_mlocked_on_free); } extern bool _debug_pagealloc_enabled_early; diff --git a/mm/internal.h b/mm/internal.h index 6614ba4ca9de..cf7799e29391 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -506,6 +506,7 @@ extern void __putback_isolated_page(struct page *page, unsigned int order, extern void memblock_free_pages(struct page *page, unsigned long pfn, unsigned int order); extern void __free_pages_core(struct page *page, unsigned int order); +extern void kernel_init_pages(struct page *page, int numpages); /* * This will have no effect, other than possibly generating a warning, if the diff --git a/mm/memory.c b/mm/memory.c index 0b92336bcebd..80944acb5b4e 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1506,6 +1506,12 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb, if (unlikely(page_mapcount(page) < 0)) print_bad_pte(vma, addr, ptent, page); } + + if (want_init_mlocked_on_free() && folio_test_mlocked(folio) && + !delay_rmap && folio_test_anon(folio)) { + kernel_init_pages(page, folio_nr_pages(folio)); + } + if (unlikely(__tlb_remove_folio_pages(tlb, page, nr, delay_rmap))) { *force_flush = true; *force_break = true; diff --git a/mm/mm_init.c b/mm/mm_init.c index d01912b8a597..2c8f3af4430f 100644 --- a/mm/mm_init.c +++ b/mm/mm_init.c @@ -2522,6 +2522,9 @@ EXPORT_SYMBOL(init_on_alloc); DEFINE_STATIC_KEY_MAYBE(CONFIG_INIT_ON_FREE_DEFAULT_ON, init_on_free); EXPORT_SYMBOL(init_on_free); +DEFINE_STATIC_KEY_MAYBE(CONFIG_INIT_MLOCKED_ON_FREE_DEFAULT_ON, init_mlocked_on_free); +EXPORT_SYMBOL(init_mlocked_on_free); + static bool _init_on_alloc_enabled_early __read_mostly = IS_ENABLED(CONFIG_INIT_ON_ALLOC_DEFAULT_ON); static int __init early_init_on_alloc(char *buf) @@ -2539,6 +2542,14 @@ static int __init early_init_on_free(char *buf) } early_param("init_on_free", early_init_on_free); +static bool _init_mlocked_on_free_enabled_early __read_mostly + = IS_ENABLED(CONFIG_INIT_MLOCKED_ON_FREE_DEFAULT_ON); +static int __init early_init_mlocked_on_free(char *buf) +{ + return kstrtobool(buf, &_init_mlocked_on_free_enabled_early); +} +early_param("init_mlocked_on_free", early_init_mlocked_on_free); + DEFINE_STATIC_KEY_MAYBE(CONFIG_DEBUG_VM, check_pages_enabled); /* @@ -2566,12 +2577,21 @@ static void __init mem_debugging_and_hardening_init(void) } #endif - if ((_init_on_alloc_enabled_early || _init_on_free_enabled_early) && + if ((_init_on_alloc_enabled_early || _init_on_free_enabled_early || + _init_mlocked_on_free_enabled_early) && page_poisoning_requested) { pr_info("mem auto-init: CONFIG_PAGE_POISONING is on, " - "will take precedence over init_on_alloc and init_on_free\n"); + "will take precedence over init_on_alloc, init_on_free " + "and init_mlocked_on_free\n"); _init_on_alloc_enabled_early = false; _init_on_free_enabled_early = false; + _init_mlocked_on_free_enabled_early = false; + } + + if (_init_mlocked_on_free_enabled_early && _init_on_free_enabled_early) { + pr_info("mem auto-init: init_on_free is on, " + "will take precedence over init_mlocked_on_free\n"); + _init_mlocked_on_free_enabled_early = false; } if (_init_on_alloc_enabled_early) { @@ -2588,9 +2608,17 @@ static void __init mem_debugging_and_hardening_init(void) static_branch_disable(&init_on_free); } - if (IS_ENABLED(CONFIG_KMSAN) && - (_init_on_alloc_enabled_early || _init_on_free_enabled_early)) - pr_info("mem auto-init: please make sure init_on_alloc and init_on_free are disabled when running KMSAN\n"); + if (_init_mlocked_on_free_enabled_early) { + want_check_pages = true; + static_branch_enable(&init_mlocked_on_free); + } else { + static_branch_disable(&init_mlocked_on_free); + } + + if (IS_ENABLED(CONFIG_KMSAN) && (_init_on_alloc_enabled_early || + _init_on_free_enabled_early || _init_mlocked_on_free_enabled_early)) + pr_info("mem auto-init: please make sure init_on_alloc, init_on_free and " + "init_mlocked_on_free are disabled when running KMSAN\n"); #ifdef CONFIG_DEBUG_PAGEALLOC if (debug_pagealloc_enabled()) { @@ -2629,9 +2657,10 @@ static void __init report_meminit(void) else stack = "off"; - pr_info("mem auto-init: stack:%s, heap alloc:%s, heap free:%s\n", + pr_info("mem auto-init: stack:%s, heap alloc:%s, heap free:%s, mlocked free:%s\n", stack, want_init_on_alloc(GFP_KERNEL) ? "on" : "off", - want_init_on_free() ? "on" : "off"); + want_init_on_free() ? "on" : "off", + want_init_mlocked_on_free() ? "on" : "off"); if (want_init_on_free()) pr_info("mem auto-init: clearing system memory may take some time...\n"); } diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 47ab0297838a..e030ccf9d5bc 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -1032,7 +1032,7 @@ static inline bool should_skip_kasan_poison(struct page *page) return page_kasan_tag(page) == KASAN_TAG_KERNEL; } -static void kernel_init_pages(struct page *page, int numpages) +void kernel_init_pages(struct page *page, int numpages) { int i; diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening index 2cff851ebfd7..effbf5982be1 100644 --- a/security/Kconfig.hardening +++ b/security/Kconfig.hardening @@ -255,6 +255,21 @@ config INIT_ON_FREE_DEFAULT_ON touching "cold" memory areas. Most cases see 3-5% impact. Some synthetic workloads have measured as high as 8%. +config INIT_MLOCKED_ON_FREE_DEFAULT_ON + bool "Enable mlocked memory zeroing on free" + depends on !KMSAN + help + This config has the effect of setting "init_mlocked_on_free=1" + on the kernel command line. If it is enabled, all mlocked process + memory is zeroed when freed. This restriction to mlocked memory + improves performance over "init_on_free" but can still be used to + protect confidential data like key material from content exposures + to other processes, as well as live forensics and cold boot attacks. + Any non-mlocked memory is not cleared before it is reassigned. This + configuration can be overwritten by setting "init_mlocked_on_free=0" + on the command line. The "init_on_free" boot option takes + precedence over "init_mlocked_on_free". + config CC_HAS_ZERO_CALL_USED_REGS def_bool $(cc-option,-fzero-call-used-regs=used-gpr) # https://github.com/ClangBuiltLinux/linux/issues/1766