From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-x530.google.com (mail-pg1-x530.google.com [IPv6:2607:f8b0:4864:20::530]) by sourceware.org (Postfix) with ESMTPS id BF4623839C6F for ; Wed, 11 May 2022 19:43:02 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org BF4623839C6F Received: by mail-pg1-x530.google.com with SMTP id 7so2644359pga.12 for ; Wed, 11 May 2022 12:43:02 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=6hpT/N3+fWLBYXNHTXGg0U/1wZyaIgcIteGZ5B8aMnI=; b=LDxdY3Y5gcd35B8LDdzeTGc/7K2hwNGVeJBt7LAS6xgkO3n1pdebuCp9zJAEGcSyGn j5vYhsINZqQQ5Z/pMXz3eE2Hfj7tuo0tRa9MHVh16ytYtEWvtGBayF1FXbZ6AUQMDKvi zjb87khTZkCBe5577V9NtkUZhilmiaq81ag/MYR/Edb8d4XRhsZgKwdSBXWnIJ+7IT4F Jvj2HbGzyqLf76O96cAYEpf5k+jiAf5V/LRxTR+LWtgToLAND03dMqy/nkQfgb75aBwS Ey44M50swvM5xQRJbvQ1WTQZEoaVTEHRMnJkqaU9piZCAGvLZhf3DaLxR62d/Tr8Hyww 5xeQ== X-Gm-Message-State: AOAM533b/bKr97oIt1xMzCfP0HGyC+5Xj/NpyGTiA+qOV+Z93VvA/dIS tjEAk2Q98ah4wu5I93J51wTtJvqkUB7goQ== X-Google-Smtp-Source: ABdhPJyJtvjNTF8htqzeKx9rUAWYE9EyWVjU7iWqztURZED8AeXsmXIMa8z1mouAyFCmVXTZ2KXeeg== X-Received: by 2002:a62:f20d:0:b0:50d:6961:7b75 with SMTP id m13-20020a62f20d000000b0050d69617b75mr26693569pfh.19.1652298181553; Wed, 11 May 2022 12:43:01 -0700 (PDT) Received: from google.com ([2620:15c:2ce:200:3228:6d2d:ebc8:7bc1]) by smtp.gmail.com with ESMTPSA id p3-20020a170902780300b0015e8d4eb293sm2285478pll.221.2022.05.11.12.43.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 May 2022 12:43:01 -0700 (PDT) Date: Wed, 11 May 2022 12:42:58 -0700 From: Fangrui Song To: "H.J. Lu" Cc: Florian Weimer , GNU C Library , Binutils Subject: Re: PT_GNU_RELRO is somewhat broken Message-ID: <20220511194258.uxbki5opu6mcvdvt@google.com> References: <871qx0dmz5.fsf@oldenburg.str.redhat.com> <20220511181704.y4pldvlqnbix3p53@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Spam-Status: No, score=-20.1 required=5.0 tests=BAYES_00, DKIMWL_WL_MED, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, ENV_AND_HDR_SPF_MATCH, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE, USER_IN_DEF_DKIM_WL, USER_IN_DEF_SPF_WL 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: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 11 May 2022 19:43:04 -0000 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 >> >> (“Read-Only after RELocation”). >> >> >> >> glibc has this code in the dynamic loader in elf/dl-reloc.c: >> >> >> >> | void >> >> | _dl_protect_relro (struct link_map *l) >> >> | { >> >> | ElfW(Addr) start = ALIGN_DOWN((l->l_addr >> >> | + l->l_relro_addr), >> >> | GLRO(dl_pagesize)); >> >> | ElfW(Addr) end = ALIGN_DOWN((l->l_addr >> >> | + l->l_relro_addr >> >> | + l->l_relro_size), >> >> | GLRO(dl_pagesize)); >> >> | if (start != end >> >> | && __mprotect ((void *) start, end - start, PROT_READ) < 0) >> >> | { >> >> | static const char errstring[] = 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 reported 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 protection. >> >> But _dl_relocate_object has a bug: to be conservative, it would have 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 boundary. >> >> 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 Align >> >> | PHDR 0x0000000000000040 0x0000000000000040 0x0000000000000040 >> >> | 0x00000000000002d8 0x00000000000002d8 R 0x8 >> >> | INTERP 0x0000000000000318 0x0000000000000318 0x0000000000000318 >> >> | 0x000000000000001c 0x000000000000001c R 0x1 >> >> | [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2] >> >> | LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000 >> >> | 0x0000000000000ff8 0x0000000000000ff8 R 0x1000 >> >> | LOAD 0x0000000000001000 0x0000000000001000 0x0000000000001000 >> >> | 0x00000000000029a1 0x00000000000029a1 R E 0x1000 >> >> | LOAD 0x0000000000004000 0x0000000000004000 0x0000000000004000 >> >> | 0x0000000000000d38 0x0000000000000d38 R 0x1000 >> >> | LOAD 0x0000000000005c78 0x0000000000006c78 0x0000000000006c78 >> >> | 0x0000000000000390 0x00000000000003a0 RW 0x1000 >> >> | DYNAMIC 0x0000000000005c90 0x0000000000006c90 0x0000000000006c90 >> >> | 0x00000000000001f0 0x00000000000001f0 RW 0x8 >> >> | NOTE 0x0000000000000338 0x0000000000000338 0x0000000000000338 >> >> | 0x0000000000000050 0x0000000000000050 R 0x8 >> >> | NOTE 0x0000000000000388 0x0000000000000388 0x0000000000000388 >> >> | 0x0000000000000044 0x0000000000000044 R 0x4 >> >> | GNU_PROPERTY 0x0000000000000338 0x0000000000000338 0x0000000000000338 >> >> | 0x0000000000000050 0x0000000000000050 R 0x8 >> >> | GNU_EH_FRAME 0x00000000000049c4 0x00000000000049c4 0x00000000000049c4 >> >> | 0x000000000000007c 0x000000000000007c R 0x4 >> >> | GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 >> >> | 0x0000000000000000 0x0000000000000000 RW 0x10 >> >> | GNU_RELRO 0x0000000000005c78 0x0000000000006c78 0x0000000000006c78 >> >> | 0x0000000000000388 0x0000000000000388 R 0x1 >> >> | […] >> >> >> >> The virtual address for PT_GNU_RELRO is 0x388, which is definitely not >> >> aligned to a 4K page. (0x388 + 0x6c78 == 0x7000, so at least the end >> >> address is aligned.) In practice, this seems to work because the RELRO >> >> area seems to be at the start of the RW LOAD segment, so we can safely >> >> flip the slack space at the start of the page to RO. It still looks >> >> like a major wart to me, though. >> > >> >After relocation, we change the end of the RO segment (aligned down from >> >the beginning of the RELRO area) to the end of the RELRO segment to RO. >> >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 protection. >> > >> >> 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) = p_vaddr(first RW PT_LOAD); https://sourceware.org/binutils/docs/ld/Builtin-Functions.html DATA_SEGMENT_RELRO_END is designed this way >> * p_vaddr(PT_GNU_RELRO) + p_memsz(PT_GNU_RELRO) is aligned by common-page-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=2097152 -z separate-code` aligns the end of PT_GNU_RELRO to common-page-size for an executable (0xaa82000, not a multiple of 2097152.)