Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack

[H.J. Lu]

On Thu, Jun 7, 2018 at 3:02 PM, H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> On Thu, Jun 7, 2018 at 1:33 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>>>
>>> On Thu, 2018-06-07 at 11:48 -0700, Andy Lutomirski wrote:
>>> > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>>> > >
>>> > > The following operations are provided.
>>> > >
>>> > > ARCH_CET_STATUS:
>>> > >         return the current CET status
>>> > >
>>> > > ARCH_CET_DISABLE:
>>> > >         disable CET features
>>> > >
>>> > > ARCH_CET_LOCK:
>>> > >         lock out CET features
>>> > >
>>> > > ARCH_CET_EXEC:
>>> > >         set CET features for exec()
>>> > >
>>> > > ARCH_CET_ALLOC_SHSTK:
>>> > >         allocate a new shadow stack
>>> > >
>>> > > ARCH_CET_PUSH_SHSTK:
>>> > >         put a return address on shadow stack
>>> > >

>> And why do we need ARCH_CET_EXEC?
>>
>> For background, I really really dislike adding new state that persists
>> across exec().  It's nice to get as close to a clean slate as possible
>> after exec() so that programs can run in a predictable environment.
>> exec() is also a security boundary, and anything a task can do to
>> affect itself after exec() needs to have its security implications
>> considered very carefully.  (As a trivial example, you should not be
>> able to use cetcmd ... sudo [malicious options here] to cause sudo to
>> run with CET off and then try to exploit it via the malicious options.
>>
>> If a shutoff is needed for testing, how about teaching ld.so to parse
>> LD_CET=no or similar and protect it the same way as LD_PRELOAD is
>> protected.  Or just do LD_PRELOAD=/lib/libdoesntsupportcet.so.
>>
>
> I will take a look.

We can use LD_CET to turn off CET.   Since most of legacy binaries
are compatible with shadow stack,  ARCH_CET_EXEC can be used
to turn on shadow stack on legacy binaries:

[hjl@gnu-cet-1 glibc]$ readelf -n /bin/ls| head -10

Displaying notes found in: .note.ABI-tag
  Owner                 Data size Description
  GNU                  0x00000010 NT_GNU_ABI_TAG (ABI version tag)
    OS: Linux, ABI: 3.2.0

Displaying notes found in: .note.gnu.property
  Owner                 Data size Description
  GNU                  0x00000020 NT_GNU_PROPERTY_TYPE_0
      Properties: x86 ISA used:
[hjl@gnu-cet-1 glibc]$ cetcmd --on -- /bin/ls /
Segmentation fault
[hjl@gnu-cet-1 glibc]$ cetcmd --on -f shstk -- /bin/ls /
bin   dev  export  lib   libx32      media  mnt  opt root  sbin  sys  usr
boot  etc  home    lib64  lost+found  misc   net  proc run   srv   tmp  var
[hjl@gnu-cet-1 glibc]$ cetcmd --on -f ibt -- /bin/ls /
Segmentation fault
[hjl@gnu-cet-1 glibc]$

-- 
H.J.