From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pj1-x1030.google.com (mail-pj1-x1030.google.com [IPv6:2607:f8b0:4864:20::1030]) by sourceware.org (Postfix) with ESMTPS id 67C70383642F; Wed, 11 May 2022 19:55:44 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 67C70383642F Received: by mail-pj1-x1030.google.com with SMTP id n10so3181029pjh.5; Wed, 11 May 2022 12:55:44 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-transfer-encoding; bh=9c4rFaFvMYYw9h25jWFib/ljNjSmGyf6GIwWDhYCFMM=; b=As6vIvwp9eVt9nufwKG9SO0POjiyyEzmuhicl8qqJgill3iAhJHZEnX/WwKX9lzvIz TEHDuOHfIsa8JVayp2luznmsgzOe5ohzBzu/I2MntXVrG7kmo2Wg+2CIoO91fXDyUs8q ExUXxYDL1Wp0M1oMC1dmwj42/ZOMbAj66si0GEl29FQouBkljlVRFe2Y07WdEOlerUJ9 9Ttx8kgtqCUtdUH3ILqbpSFlKkANMUVZpOtDZg4PHdAy4T3w7atEr/2tTTRNypFmwsDO 3DmTFGtxUBJfFZL9Txs+3QyNGKVoRAp1FTIUiL86e1T068DROzJLUuHmhs//kQLKyrad DvPw== X-Gm-Message-State: AOAM5325BXpPBJKKca3jXmOkLy7twW0egRtc2qQ3sK1wvq9BVKRY6TZ9 e998LBTFHACaAd6pZK7cahjW6/D1gLJHyG0JG2E= X-Google-Smtp-Source: ABdhPJwkFAKd9bzCTKrnJxn0gtTWrynfEmGFeGrAGPfqDJlTI+dPcJNPJJL8y1OGHCmnaYeKK3RR9X/GWxqneZkqOxw= X-Received: by 2002:a17:903:1108:b0:156:73a7:7c1 with SMTP id n8-20020a170903110800b0015673a707c1mr26445220plh.101.1652298943450; Wed, 11 May 2022 12:55:43 -0700 (PDT) MIME-Version: 1.0 References: <871qx0dmz5.fsf@oldenburg.str.redhat.com> <20220511181704.y4pldvlqnbix3p53@google.com> <20220511194258.uxbki5opu6mcvdvt@google.com> In-Reply-To: <20220511194258.uxbki5opu6mcvdvt@google.com> From: "H.J. Lu" Date: Wed, 11 May 2022 12:55:07 -0700 Message-ID: Subject: Re: PT_GNU_RELRO is somewhat broken To: Fangrui Song Cc: Florian Weimer , GNU C Library , Binutils Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-3019.1 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: binutils@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Binutils mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 11 May 2022 19:55:46 -0000 On Wed, May 11, 2022 at 12:43 PM Fangrui Song wrote: > > > On 2022-05-11, H.J. Lu wrote: > >On Wed, May 11, 2022 at 11:17 AM Fangrui Song wrote= : > >> > >> On 2022-05-11, H.J. Lu via Libc-alpha wrote: > >> >On Wed, May 11, 2022 at 9:59 AM Florian Weimer via Libc-alpha > >> > wrote: > >> >> > >> >> PT_GNU_RELRO is supposed to identify a region in the process image = which > >> >> has to be flipped to PROT_READ (only) permission after relocation > >> >> (=E2=80=9CRead-Only after RELocation=E2=80=9D). > >> >> > >> >> glibc has this code in the dynamic loader in elf/dl-reloc.c: > >> >> > >> >> | void > >> >> | _dl_protect_relro (struct link_map *l) > >> >> | { > >> >> | ElfW(Addr) start =3D ALIGN_DOWN((l->l_addr > >> >> | + l->l_relro_addr), > >> >> | GLRO(dl_pagesize)); > >> >> | ElfW(Addr) end =3D ALIGN_DOWN((l->l_addr > >> >> | + l->l_relro_addr > >> >> | + l->l_relro_size), > >> >> | GLRO(dl_pagesize)); > >> >> | if (start !=3D end > >> >> | && __mprotect ((void *) start, end - start, PROT_READ) < 0) > >> >> | { > >> >> | static const char errstring[] =3D N_("\ > >> >> | cannot apply additional memory protection after relocation"); > >> >> | _dl_signal_error (errno, l->l_name, NULL, errstring); > >> >> | } > >> >> | } > >> >> > >> >> I assume the intent is to conservatively apply the largest possible > >> >> RELRO region given GLRO(dl_pagesize), the run-time page size report= ed by > >> >> the kernel. If the binary is built to a smaller page size (to save= disk > >> >> space), glibc can still load it, but apply only some RELRO protecti= on. > >> >> But _dl_relocate_object has a bug: to be conservative, it would hav= e to > >> >> use ALGIN_UP for the start (lower) address of the range. > >> >> > >> >> But it turns out we can't make this change without incurring a loss= of > >> >> hardening: BFD ld does not align the start address to a page bounda= ry. > >> >> For example, /bin/true in Fedora 35 x86-64 has this: > >> >> > >> >> | $ readelf -l /bin/true > >> >> | > >> >> | Elf file type is DYN (Position-Independent Executable file) > >> >> | Entry point 0x1960 > >> >> | There are 13 program headers, starting at offset 64 > >> >> | > >> >> | Program Headers: > >> >> | Type Offset VirtAddr PhysAddr > >> >> | FileSiz MemSiz Flags Al= ign > >> >> | PHDR 0x0000000000000040 0x0000000000000040 0x00000000= 00000040 > >> >> | 0x00000000000002d8 0x00000000000002d8 R 0x= 8 > >> >> | INTERP 0x0000000000000318 0x0000000000000318 0x00000000= 00000318 > >> >> | 0x000000000000001c 0x000000000000001c R 0x= 1 > >> >> | [Requesting program interpreter: /lib64/ld-linux-x86-64.so.= 2] > >> >> | LOAD 0x0000000000000000 0x0000000000000000 0x00000000= 00000000 > >> >> | 0x0000000000000ff8 0x0000000000000ff8 R 0x= 1000 > >> >> | LOAD 0x0000000000001000 0x0000000000001000 0x00000000= 00001000 > >> >> | 0x00000000000029a1 0x00000000000029a1 R E 0x= 1000 > >> >> | LOAD 0x0000000000004000 0x0000000000004000 0x00000000= 00004000 > >> >> | 0x0000000000000d38 0x0000000000000d38 R 0x= 1000 > >> >> | LOAD 0x0000000000005c78 0x0000000000006c78 0x00000000= 00006c78 > >> >> | 0x0000000000000390 0x00000000000003a0 RW 0x= 1000 > >> >> | DYNAMIC 0x0000000000005c90 0x0000000000006c90 0x00000000= 00006c90 > >> >> | 0x00000000000001f0 0x00000000000001f0 RW 0x= 8 > >> >> | NOTE 0x0000000000000338 0x0000000000000338 0x00000000= 00000338 > >> >> | 0x0000000000000050 0x0000000000000050 R 0x= 8 > >> >> | NOTE 0x0000000000000388 0x0000000000000388 0x00000000= 00000388 > >> >> | 0x0000000000000044 0x0000000000000044 R 0x= 4 > >> >> | GNU_PROPERTY 0x0000000000000338 0x0000000000000338 0x00000000= 00000338 > >> >> | 0x0000000000000050 0x0000000000000050 R 0x= 8 > >> >> | GNU_EH_FRAME 0x00000000000049c4 0x00000000000049c4 0x00000000= 000049c4 > >> >> | 0x000000000000007c 0x000000000000007c R 0x= 4 > >> >> | GNU_STACK 0x0000000000000000 0x0000000000000000 0x00000000= 00000000 > >> >> | 0x0000000000000000 0x0000000000000000 RW 0x= 10 > >> >> | GNU_RELRO 0x0000000000005c78 0x0000000000006c78 0x00000000= 00006c78 > >> >> | 0x0000000000000388 0x0000000000000388 R 0x= 1 > >> >> | [=E2=80=A6] > >> >> > >> >> The virtual address for PT_GNU_RELRO is 0x388, which is definitely = not > >> >> aligned to a 4K page. (0x388 + 0x6c78 =3D=3D 0x7000, so at least t= he end > >> >> address is aligned.) In practice, this seems to work because the R= ELRO > >> >> area seems to be at the start of the RW LOAD segment, so we can saf= ely > >> >> flip the slack space at the start of the page to RO. It still look= s > >> >> like a major wart to me, though. > >> > > >> >After relocation, we change the end of the RO segment (aligned down f= rom > >> >the beginning of the RELRO area) to the end of the RELRO segment to R= O. > >> >Since the end of the RELRO segment must be aligned to the page size, > >> >ALIGN_DOWN on the end of the RELRO segment doesn't lose any protectio= n. > >> > > >> >> Any suggestions what should we do to fix this properly, mainly for > >> >> targets that have varying page size in practice? > >> > > >> >The end of the RELRO segment should be aligned to the maximum page > >> >size. > >> > > >> > >> PT_GNU_RELRO is designed/implemented this way: > >> > >> * there can be at most one PT_GNU_RELRO > >> * p_vaddr(PT_GNU_RELRO) =3D p_vaddr(first RW PT_LOAD); https://sourcew= are.org/binutils/docs/ld/Builtin-Functions.html DATA_SEGMENT_RELRO_END is d= esigned this way > >> * p_vaddr(PT_GNU_RELRO) + p_memsz(PT_GNU_RELRO) is aligned by common-p= age-size. comon page size is chosen probably because of less waste > > > >ld aligns DATA_SEGMENT_RELRO_END to the maximum page size. > > Is p_vaddr(PT_GNU_RELRO) + p_memsz(PT_GNU_RELRO) aligned to max-page-size= for non-x86 ports? > I know some changes have been made in binutils in recent months, but > don't know the exact state. > If so, the security looks good to me. > > With ld 2.38's x86-64 port, `-z max-page-size=3D2097152 -z separate-code` > aligns the end of PT_GNU_RELRO to common-page-size for an executable > (0xaa82000, not a multiple of 2097152.) It is fixed by: https://sourceware.org/bugzilla/show_bug.cgi?id=3D28824 --=20 H.J.