Re: [RFC PATCH v6 22/26] x86/cet/shstk: ELF header parsing of Shadow Stack

Dave Martin
2019-04-25T16:11:20Z

On Thu, Apr 25, 2019 at 04:35:48PM +0100, Dave Martin wrote:
> On Thu, Apr 25, 2019 at 08:14:52AM -0700, Yu-cheng Yu wrote:
> > On Thu, 2019-04-25 at 12:02 +0100, Dave Martin wrote:
> > > On Mon, Nov 19, 2018 at 01:48:05PM -0800, Yu-cheng Yu wrote:
> > > > Look in .note.gnu.property of an ELF file and check if Shadow Stack needs
> > > > to be enabled for the task.
> > >
> > > What's the status of this series? I don't see anything in linux-next
> > > yet.
> > >
> > > For describing ELF features, Arm has recently adopted
> > > NT_GNU_PROPERTY_TYPE_0, with properties closely modelled on
> > > GNU_PROPERTY_X86_FEATURE_1_AND etc. [1]
> > >
> > > So, arm64 will be need something like this patch for supporting new
> > > features (such as the Branch Target Identification feature of ARMv8.5-A
> > > [2]).
> > >
> > > If this series isn't likely to merge soon, can we split this patch into
> > > generic and x86-specific parts and handle them separately?
> > >
> > > It would be good to see the generic ELF note parsing move to common
> > > code -- I'll take a look and comment in more detail.
> >
> > Yes, I will work on that.
>
> Thanks. I may try to hack something in the meantime based on your
> patch.
>
> One other question: according to the draft spec at
> https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI, it
> looks like the .note.gnu.property section is supposed to be marked with
> SHF_ALLOC in object files.
>
> I think that means that the linker will map it with a PT_LOAD entry in
> the program header table in addition to the PT_NOTE that describes the
> location of the note. I need to check what the toolchain actually
> does.
>
> If so, can we simply rely on the notes being already mapped, rather than
> needing to do additional I/O on the ELF file to fetch the notes?

[...]

BTW, it looks like this holds true for AArch64 (see below).

Providing this also works on other arches, I think we can just pick
PT_GNU_PROPERTY out of the program headers and rely on the
corresponding note being already mapped by the existing binfmt_elf
code.

Cheers
---Dave

--8<--

$ echo 'void f(void) { }' | \
aarch64-linux-gnu-gcc -v -nostdlib -Wl,-ef \
-mbranch-protection=standard -o /tmp/x -x c - && \
aarch64-linux-gnu-readelf -nl /tmp/x

[...]

gcc version 9.0.1 20190425 (experimental) (GCC)

[...]

GNU assembler version 2.32.51 (aarch64-linux-gnu) using BFD version (GNU Binutils) 2.32.51.20190425

[...]

Elf file type is EXEC (Executable file)
Entry point 0x400178
There are 5 program headers, starting at offset 64

Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x00000000000001c0 0x00000000000001c0 R E 0x10000
NOTE 0x0000000000000158 0x0000000000400158 0x0000000000400158
0x0000000000000020 0x0000000000000020 R 0x8
GNU_PROPERTY 0x0000000000000158 0x0000000000400158 0x0000000000400158
0x0000000000000020 0x0000000000000020 R 0x8
GNU_EH_FRAME 0x0000000000000184 0x0000000000400184 0x0000000000400184
0x0000000000000014 0x0000000000000014 R 0x4
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RW 0x10

Section to Segment mapping:
Segment Sections...
00 .note.gnu.property .text .eh_frame_hdr .eh_frame
01 .note.gnu.property
02 .note.gnu.property
03 .eh_frame_hdr
04

Displaying notes found in: .note.gnu.property
Owner Data size Description
GNU 0x00000010 NT_GNU_PROPERTY_TYPE_0
Properties: AArch64 feature: BTI, PAC