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Tue, 12 Feb 2019 12:38:48 +0100 (CET) Subject: Re: [PATCH v4 3/3] powerpc/32: Add KASAN support To: Daniel Axtens , Andrey Ryabinin , Andrey Konovalov Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Nicholas Piggin , "Aneesh Kumar K.V" , Alexander Potapenko , Dmitry Vyukov , Linux Memory Management List , PowerPC , LKML , kasan-dev References: <1f5629e03181d0e30efc603f00dad78912991a45.1548166824.git.christophe.leroy@c-s.fr> <87ef8i45km.fsf@dja-thinkpad.axtens.net> <69720148-fd19-0810-5a1d-96c45e2ec00c@c-s.fr> <805fbf9d-a10f-03e0-aa52-6f6bd16059b9@virtuozzo.com> <87imxpak4r.fsf@linkitivity.dja.id.au> From: Christophe Leroy Message-ID: Date: Tue, 12 Feb 2019 12:38:48 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.5.0 MIME-Version: 1.0 In-Reply-To: <87imxpak4r.fsf@linkitivity.dja.id.au> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Le 12/02/2019 à 02:08, Daniel Axtens a écrit : > Andrey Ryabinin writes: > >> On 2/11/19 3:25 PM, Andrey Konovalov wrote: >>> On Sat, Feb 9, 2019 at 12:55 PM christophe leroy >>> wrote: >>>> >>>> Hi Andrey, >>>> >>>> Le 08/02/2019 à 18:40, Andrey Konovalov a écrit : >>>>> On Fri, Feb 8, 2019 at 6:17 PM Christophe Leroy wrote: >>>>>> >>>>>> Hi Daniel, >>>>>> >>>>>> Le 08/02/2019 à 17:18, Daniel Axtens a écrit : >>>>>>> Hi Christophe, >>>>>>> >>>>>>> I've been attempting to port this to 64-bit Book3e nohash (e6500), >>>>>>> although I think I've ended up with an approach more similar to Aneesh's >>>>>>> much earlier (2015) series for book3s. >>>>>>> >>>>>>> Part of this is just due to the changes between 32 and 64 bits - we need >>>>>>> to hack around the discontiguous mappings - but one thing that I'm >>>>>>> particularly puzzled by is what the kasan_early_init is supposed to do. >>>>>> >>>>>> It should be a problem as my patch uses a 'for_each_memblock(memory, >>>>>> reg)' loop. >>>>>> >>>>>>> >>>>>>>> +void __init kasan_early_init(void) >>>>>>>> +{ >>>>>>>> + unsigned long addr = KASAN_SHADOW_START; >>>>>>>> + unsigned long end = KASAN_SHADOW_END; >>>>>>>> + unsigned long next; >>>>>>>> + pmd_t *pmd = pmd_offset(pud_offset(pgd_offset_k(addr), addr), addr); >>>>>>>> + int i; >>>>>>>> + phys_addr_t pa = __pa(kasan_early_shadow_page); >>>>>>>> + >>>>>>>> + BUILD_BUG_ON(KASAN_SHADOW_START & ~PGDIR_MASK); >>>>>>>> + >>>>>>>> + if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE)) >>>>>>>> + panic("KASAN not supported with Hash MMU\n"); >>>>>>>> + >>>>>>>> + for (i = 0; i < PTRS_PER_PTE; i++) >>>>>>>> + __set_pte_at(&init_mm, (unsigned long)kasan_early_shadow_page, >>>>>>>> + kasan_early_shadow_pte + i, >>>>>>>> + pfn_pte(PHYS_PFN(pa), PAGE_KERNEL_RO), 0); >>>>>>>> + >>>>>>>> + do { >>>>>>>> + next = pgd_addr_end(addr, end); >>>>>>>> + pmd_populate_kernel(&init_mm, pmd, kasan_early_shadow_pte); >>>>>>>> + } while (pmd++, addr = next, addr != end); >>>>>>>> +} >>>>>>> >>>>>>> As far as I can tell it's mapping the early shadow page, read-only, over >>>>>>> the KASAN_SHADOW_START->KASAN_SHADOW_END range, and it's using the early >>>>>>> shadow PTE array from the generic code. >>>>>>> >>>>>>> I haven't been able to find an answer to why this is in the docs, so I >>>>>>> was wondering if you or anyone else could explain the early part of >>>>>>> kasan init a bit better. >>>>>> >>>>>> See https://www.kernel.org/doc/html/latest/dev-tools/kasan.html for an >>>>>> explanation of the shadow. >>>>>> >>>>>> When shadow is 0, it means the memory area is entirely accessible. >>>>>> >>>>>> It is necessary to setup a shadow area as soon as possible because all >>>>>> data accesses check the shadow area, from the begining (except for a few >>>>>> files where sanitizing has been disabled in Makefiles). >>>>>> >>>>>> Until the real shadow area is set, all access are granted thanks to the >>>>>> zero shadow area beeing for of zeros. >>>>> >>>>> Not entirely correct. kasan_early_init() indeed maps the whole shadow >>>>> memory range to the same kasan_early_shadow_page. However as kernel >>>>> loads and memory gets allocated this shadow page gets rewritten with >>>>> non-zero values by different KASAN allocator hooks. Since these values >>>>> come from completely different parts of the kernel, but all land on >>>>> the same page, kasan_early_shadow_page's content can be considered >>>>> garbage. When KASAN checks memory accesses for validity it detects >>>>> these garbage shadow values, but doesn't print any reports, as the >>>>> reporting routine bails out on the current->kasan_depth check (which >>>>> has the value of 1 initially). Only after kasan_init() completes, when >>>>> the proper shadow memory is mapped, current->kasan_depth gets set to 0 >>>>> and we start reporting bad accesses. >>>> >>>> That's surprising, because in the early phase I map the shadow area >>>> read-only, so I do not expect it to get modified unless RO protection is >>>> failing for some reason. >>> >>> Actually it might be that the allocator hooks don't modify shadow at >>> this point, as the allocator is not yet initialized. However stack >>> should be getting poisoned and unpoisoned from the very start. But the >>> generic statement that early shadow gets dirtied should be correct. >>> Might it be that you don't use stack instrumentation? >>> >> >> Yes, stack instrumentation is not used here, because shadow offset which we pass to >> the -fasan-shadow-offset= cflag is not specified here. So the logic in scrpits/Makefile.kasan >> just fallbacks to CFLAGS_KASAN_MINIMAL, which is outline and without stack instrumentation. >> >> Christophe, you can specify KASAN_SHADOW_OFFSET either in Kconfig (e.g. x86_64) or >> in Makefile (e.g. arm64). And make early mapping writable, because compiler generated code will write >> to shadow memory in function prologue/epilogue. > > Hmm. Is this limitation just that compilers have not implemented > out-of-line support for stack instrumentation, or is there a deeper > reason that stack/global instrumentation relies upon inline > instrumentation? No, it looks like as soon as we define KASAN_SHADOW_OFFSET in Makefile in addition to asm/kasan.h, stack instrumentation works with out-of-line. I'll send series v5 soon. Christophe > > I ask because it's very common on ppc64 to have the virtual address > space split up into discontiguous blocks. I know this means we lose > inline instrumentation, but I didn't realise we'd also lose stack and > global instrumentation... > > I wonder if it would be worth, in the distant future, trying to > implement a smarter scheme in compilers where we could insert more > complex inline mapping schemes. > > Regards, > Daniel >