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

[H.J. Lu]

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
>> > >
>> > > ARCH_CET_ALLOC_SHSTK and ARCH_CET_PUSH_SHSTK are intended only for
>> > > the implementation of GLIBC ucontext related APIs.
>> >
>> > Please document exactly what these all do and why.  I don't understand
>> > what purpose ARCH_CET_LOCK and ARCH_CET_EXEC serve.  CET is opt in for
>> > each ELF program, so I think there should be no need for a magic
>> > override.
>>
>> CET is initially enabled if the loader has CET capability.  Then the
>> loader decides if the application can run with CET.  If the application
>> cannot run with CET (e.g. a dependent library does not have CET), then
>> the loader turns off CET before passing to the application.  When the
>> loader is done, it locks out CET and the feature cannot be turned off
>> anymore until the next exec() call.
>
> Why is the lockout necessary?  If user code enables CET and tries to
> run code that doesn't support CET, it will crash.  I don't see why we
> need special code in the kernel to prevent a user program from calling
> arch_prctl() and crashing itself.  There are already plenty of ways to
> do that :)

On CET enabled machine, not all programs nor shared libraries are
CET enabled.  But since ld.so is CET enabled, all programs start
as CET enabled.  ld.so will disable CET if a program or any of its shared
libraries aren't CET enabled.  ld.so will lock up CET once it is done CET
checking so that CET can't no longer be disabled afterwards.

>> When the next exec() is called, CET
>> feature is turned on/off based on the values set by ARCH_CET_EXEC.
>
> 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.


-- 
H.J.