# Re: new ELF marking

On Wed, Aug 9, 2017 at 11:52 AM, Kees Cook <keesc...@google.com> wrote:
> On Wed, Aug 9, 2017 at 11:46 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>> On Wed, Aug 9, 2017 at 11:26 AM, Kees Cook <keesc...@google.com> wrote:
>>> On Wed, Aug 9, 2017 at 10:16 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>> On Wed, Aug 9, 2017 at 10:07 AM, Kees Cook via binutils
>>>> <binut...@sourceware.org> wrote:
>>>>> Hi,
>>>>>
>>>>> I'd like to be able to mark an ELF binary in such a way that Linux's
>>>>> binfmt_elf.c will collapse a PIE text area into the mmap region
>>>>> (currently they are separately randomized in memory). This is desired
>>>>> by AddressSanitizer to avoid having an ASan-built binary have its text
>>>>> area moving into an unexpected location[1] (ASLR is still desired, but
>>>>> doesn't need to have a PIE/mmap split).
>>>>>
>>>>> I see a few ways:
>>>>>
>>>>> (NT_GNU_EXEC_FLAGS), though notes tend to be strings...
>>>>>
>>>>> - Add a new Program Header (GNU_EXEC_FLAGS), which is similar to how
>>>>> GNU_STACK and GNU_RELRO were handled. This could sort of be like NOTE
>>>>> except just lots of bit flags.
>>>>>
>>>>> - Use a filesystem xattr. This is fragile, in the case of copying
>>>>> binaries between systems or filesystems.
>>>>>
>>>>> Thoughts?
>>>>>
>>>>>
>>>>
>>>> Why don't you use  NT_GNU_PROPERTY_TYPE_0?
>>>
>>> Ah, interesting. I hadn't seen this before. Docs I found were:
>>> https://github.com/hjl-tools/linux-abi/commit/a24f6898c4172e09b2e476ae9f160621528a1d92
>>>
>>>  \item[pr_datasz] The size of the \code{pr_data} field.  A 4-byte
>>>    integer in the format of the target processor.
>>>  \item[pr_data] The program property descriptor.  An array of 4-byte
>>>    integers in 32-bit object or 8-byte integers in 64-bit objects, in
>>>    the format of the target processor.
>>>
>>> Is pr_data length always a multiple of 4 (or 8)? I found this language
>>
>> Yes.
>>
>>> confusing, given that pr_datasz doesn't mention this.
>
> Why does pr_padding exist if pr_data is always the correct multiple already?
>
>>>
>>> Also, given the definition, should the kernel examine these, or should
>>> it remain limited to the runtimer loader?
>>
>> Both kernel and run-time loaders should check it.  I am working on
>> static PIE, which is loaded by kernel.
>
> What kernel-support is needed for static PIE?

Nothing so far:

[hjl@gnu-6 elf]$readelf -l ./sln Elf file type is DYN (Shared object file) Entry point 0x8830 There are 8 program headers, starting at offset 64 Program Headers: Type Offset VirtAddr PhysAddr FileSiz MemSiz Flags Align LOAD 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x00000000000a1466 0x00000000000a1466 R E 0x200000 LOAD 0x00000000000a2240 0x00000000002a2240 0x00000000002a2240 0x00000000000050a0 0x0000000000006590 RW 0x200000 DYNAMIC 0x00000000000a4d48 0x00000000002a4d48 0x00000000002a4d48 0x00000000000001a0 0x00000000000001a0 RW 0x8 NOTE 0x0000000000000200 0x0000000000000200 0x0000000000000200 0x0000000000000020 0x0000000000000020 R 0x4 TLS 0x00000000000a2240 0x00000000002a2240 0x00000000002a2240 0x0000000000000020 0x0000000000000060 R 0x8 GNU_EH_FRAME 0x0000000000096298 0x0000000000096298 0x0000000000096298 0x0000000000001a14 0x0000000000001a14 R 0x4 GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 0x0000000000000000 RW 0x10 GNU_RELRO 0x00000000000a2240 0x00000000002a2240 0x00000000002a2240 0x0000000000002dc0 0x0000000000002dc0 R 0x1 Section to Segment mapping: Segment Sections... 00 .note.ABI-tag .hash .dynsym .dynstr .rela.dyn .rela.plt .init .plt .plt.got .text __libc_freeres_fn __libc_thread_freeres_fn .fini .rodata .eh_frame_hdr .eh_frame .gcc_except_table 01 .tdata .init_array .fini_array .data.rel.ro .dynamic .got .got.plt .data __libc_subfreeres __libc_IO_vtables __libc_atexit __libc_thread_subfreeres .bss __libc_freeres_ptrs 02 .dynamic 03 .note.ABI-tag 04 .tdata .tbss 05 .eh_frame_hdr 06 07 .tdata .init_array .fini_array .data.rel.ro .dynamic .got [hjl@gnu-6 elf]$ ./sln
Usage: sln src dest|file

For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.
[hjl@gnu-6 elf]\$

>>> If the kernel should, would it be better to add
>>> GNU_PROPERTY_EXEC_FLAGS, for future bits, or should it be something
>>> like GNU_PROPERTY_NO_COPY_ON_PROTECTED with a pr_datasz == 0?
>>
>
> Okay.
>
>> Is this an output only bit?  Will it appear in an input file?
>
> I believe this would only be an output bit, but I'm not sure how it
> would be wired into binutils. Kostya, do you know any details about
> how AddressSanitizer might be able to create this ELF note?

if it is an output only bit, "-z fobar" should work, like

-z ibt                      Generate GNU_PROPERTY_X86_FEATURE_1_IBT
-z shstk                    Generate GNU_PROPERTY_X86_FEATURE_1_SHSTK

Or you can put it in the input file and linker will copy it to output, like
GNU_PROPERTY_STACK_SIZE.

>>> (And should the kernel already be parsing GNU_PROPERTY_STACK_SIZE?)
>>
>> Kernel should.
>
> Have there been patches proposed for this yet?
>

Not I am aware of.

--
H.J.