PT_GNU_RELRO is somewhat broken

Fangrui Song maskray@google.com
Wed May 11 18:17:04 GMT 2022


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
><libc-alpha@sourceware.org> 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

If the proposal is to align p_vaddr(PT_GNU_RELRO) +
p_memsz(PT_GNU_RELRO) to max page size, that will penalize the size of
many max-page-size>4096 ports with the current GNU ld section/segment
layout.  See https://sourceware.org/bugzilla/show_bug.cgi?id=24490 and
https://sourceware.org/bugzilla/show_bug.cgi?id=23704 for GNU ld's -z
separate-code complaints.

Note: ld.lld used (before 9.0.0) to place PT_GNU_RELRO in the middle of
the RW PT_LOAD.  I changed it to the start in
https://reviews.llvm.org/D58892 With the new scheme, it doesn't really
matter whether p_vaddr(PT_GNU_RELRO) + p_memsz(PT_GNU_RELRO) is aligned
to max-page-size or common-page-size: the file size does not change.


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