Re: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack register

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Apr 5, 2018 at 9:46 PM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 03/30/2018 12:41 PM, H.J. Lu wrote:
>> On Thu, Mar 29, 2018 at 1:20 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>> * H. J. Lu:
>>>
>>>> On Thu, Mar 29, 2018 at 1:15 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>>>> * H. J. Lu:
>>>>>
>>>>>> You need to make a choice.  You either don't introduce a new symbol
>>>>>> version or don't save shadow stack for thread cancellation.  You
>>>>>> can't have both.
>>>>> I don't understand.  We have room to save the shadow stack pointer in
>>>>> the existing struct.
>>>> No, we don't have room in struct pthread_unwind_buf:
>>>>
>>>> Note: There is an unused pointer space in pthread_unwind_buf_data.  But
>>>> it isn't suitable for saving and restoring shadow stack register since
>>>> x32 is a 64-bit process with 32-bit software pointer and kernel may
>>>> place x32 shadow stack above 4GB.  We need to save and restore 64-bit
>>>> shadow stack register for x32.
>>> We have for void * fields.  They are subsequently overwritten by
>>> __pthread_register_cancel.  But __sigsetjmp can write to them first
>>> without causing any harm.  We just need a private __longjmp_cancel
>>> that doesn't restore the shadow stack pointer.
>> Here is the patch which does that.   Any comments?
>
> OK, so I have reviewed https://github.com/hjl-tools/glibc/ hjl/cet/master,
> please confirm that this is the correct branch of the required implementation
> for CET. It really helps to review the rest of the patch set, you should be
> preparing this as a patch set instead of having it reviewed one-at-a-time.
> This issue was already raised in the thread with Zack.

Thanks for your feedbacks.

> Architecture:
>
> * We avoid a "flag day" with feature_1 TCB flag to switch to a new ABI,
>   which we have discussed is a fragile process which should be avoided if
>   a supportable alternative solution exists.
>
> * We avoid versioned symbols, this makes CET backportable, and this has a
>   bigger benefit for long-term stable distributions.
>
> * A key design problem has been that cancellation jump buffers within glibc
>   are truncated to optimize on-stack size, and this means that setjmp could
>   write beyond the structure because setjmp now tries to save the shadowstack
>   pointer into space that the cancellation jump buffer did not allocate.
>   For the record this optimization seems premature and I'm sad we did it, this
>   is a lesson we should learn from.
>
> * We have all agreed to the following concepts:
>
>   * The cancellation process, in particular the unwinder, never returns from
>     any of the functions we call, it just keeps calling into the unwinder to
>     jump to the next unwound cancellation function all the way to the thread
>     start routine. Therefore because we never return from one of these functions
>     we never need to restore the shadow stack, and consequently wherever it is
>     stored in the cancellation jump buffer can be overwritten if we need the
>     space (it's a dead store).
>
>     * The corollary to this is that function calls made from cancellation handlers
>       will continue to advance the shadowstack from the deepest point at which
>       cancellation is initiated from. This means that the depth of the shadowstack
>       doesn't match the depth of the real stack while we are unwinding. I don't
>       know if this will have consequences on analysis tooling or not, or debug
>       tools during unwinding. It's a fairly advanced situation and corner case,
>       and restoring the shadowstack is not useful becuase we don't need it and
>       simplifies the implementation.
>
>   * The cancellation jump buffer has private data used for chaining the cancel
>     jump buffers together such that the custom unwinder can follow them and
>     call them in sequence. This space constitutes 4 void *'s which is space
>     that setjmp can write to, because we will just overwrite it when we register
>     the cancel handler.
>
>   * If the new shadowstack-enabled setjmp stores the shadowstack pointer into
>     the space taken by the 4 void*'s then we won't overflow the stack, and we
>     don't need to change the layout of the cancellation jump buffer. The 4 void*'s
>     are sufficient, even for x32 to write a 64-bit shadow stack address.
>
> * After fixing the cancellation jump buffers the following work needs to be reviewed:
>
>   * Add feature_1 in tcb head to track CET status and make it easily available
>     to runtime for checking.
>
>   * Save and restore shadowstack in setjmp/longjmp.
>
>   * Add CET support to ld.so et. al. and track runtime status.
>
>   * Adjust vfork for shadow stack usage.
>
>   * Add ENDBR or NOTRACK where required in assembly.
>
>   * CET and makecontext incompatible.
>     - Probably need to discuss which default is appropriate.
>     - Should the user get CET automatically disabled in makecontext() et. al. silently?
>     - Should your current solution, which is to error out during the build, and require
>       flag changes, be the default? This forces the user to review the security for their
>       application.

I'd like to reserve 4 slots in ucontext for shadow stack:

https://github.com/hjl-tools/glibc/commit/9bf6aefa8fb45f8df140d42ce9cf890bb24076e1

It should be binary backward compatible.  I will investigate if there is a way
to support shadow stack with existing API.  Otherwise, we need to add a new
API for ucontext functions with shadow stack.

>   * prctl for CET.

We have been experimenting different approaches to get the best implementation.
I am expecting that this patch may change as we collect more data.

> * The work to review after this patch appears to be less contentious in terms of
>   the kinds of changes that are required. Most of the changes are internal details
>   of enabling CET and not ABI details, with the exception of the possible pain we
>   might cause with makecontext() being unsupported and what default position to take
>   there.
>
> Design:
>
> * Overall the implementation looks exactly how I might expect it to look, but some
>   of the math that places the shadowstack pointer appears to need either commenting
>   or fixing because I don't understand it. You need to make it easy for me to see
>   that we have placed the shadowstack pointer into the 4 pad words.
>
> Details:
>
> * One comment needs filling out a bit more, noted below.
>
>> 0001-x86-Use-pad-in-pthread_unwind_buf-to-preserve-shadow.patch
>>
>>
>> From f222537447f5ec879427f318b7c0396362b7453a Mon Sep 17 00:00:00 2001
>> From: "H.J. Lu" <hjl.tools@gmail.com>
>> Date: Sat, 24 Feb 2018 17:23:54 -0800
>> Subject: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack
>>  register
>>
>> The pad array in struct pthread_unwind_buf is used by setjmp to save
>> shadow stack register.  We assert that size of struct pthread_unwind_buf
>> is no less than offset of shadow stack pointer + shadow stack pointer
>> size.
>>
>
> OK.
>
>> Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
>> these with thread cancellation, call setjmp, but never return after
>> __libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
>> __libc_longjmp on x86, doesn't need to restore shadow stack register.
>
> OK.
>
>> __libc_longjmp, which is a private interface for thread cancellation
>> implementation in libpthread, is changed to call __longjmp_cancel,
>> instead of __longjmp.  __longjmp_cancel is a new internal function
>> in libc, which is similar to __longjmp, but doesn't restore shadow
>> stack register.
>
> OK. Good. I like the use of a __longjmp_cancel name to call out what's
> going on in the API (clear semantics).
>
>>
>> The compatibility longjmp and siglongjmp in libpthread.so are changed
>> to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
>> restore shadow stack register.
>
> OK.
>
>>
>> Tested with build-many-glibcs.py.
>>
>>       * nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
>>       handlers after setjmp.
>>       * setjmp/longjmp.c (__libc_longjmp): Don't define alias if
>>       defined.
>>       * sysdeps/unix/sysv/linux/x86/setjmpP.h (_JUMP_BUF_SIGSET_NSIG):
>>       Changed to 97.
>>       * sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
>>       * sysdeps/x86/__longjmp_cancel.S: New file.
>>       * sysdeps/x86/longjmp.c: Likewise.
>>       * sysdeps/x86/nptl/pt-longjmp.c: Likewise.
>
> This looks much better.
>
>> ---
>>  nptl/pthread_create.c                 |  9 ++--
>>  setjmp/longjmp.c                      |  2 +
>>  sysdeps/unix/sysv/linux/x86/setjmpP.h |  4 +-
>>  sysdeps/x86/Makefile                  |  4 ++
>>  sysdeps/x86/__longjmp_cancel.S        | 20 ++++++++
>>  sysdeps/x86/longjmp.c                 | 45 ++++++++++++++++
>>  sysdeps/x86/nptl/pt-longjmp.c         | 97 +++++++++++++++++++++++++++++++++++
>>  7 files changed, 176 insertions(+), 5 deletions(-)
>>  create mode 100644 sysdeps/x86/__longjmp_cancel.S
>>  create mode 100644 sysdeps/x86/longjmp.c
>>  create mode 100644 sysdeps/x86/nptl/pt-longjmp.c
>>
>> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
>> index caaf07c134..1c5b3780c6 100644
>> --- a/nptl/pthread_create.c
>> +++ b/nptl/pthread_create.c
>> @@ -427,12 +427,15 @@ START_THREAD_DEFN
>>       compilers without that support we do use setjmp.  */
>>    struct pthread_unwind_buf unwind_buf;
>>
>> -  /* No previous handlers.  */
>> +  int not_first_call;
>> +  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
>> +
>> +  /* No previous handlers.  NB: This must be done after setjmp since
>> +     the same space may be used by setjmp to store extra data which
>> +     should never be used by __libc_unwind_longjmp.  */
>
> Suggest:
> ~~~
> No previous handlers.  NB: This must be done after setjmp since
> the private space in the unwind jump buffer may overlap space
> used by setjmp to store extra architecture-specific information
> which is never be used by the cancellation-specific
> __libc_unwind_longjmp.
>
> The private space is allowed to overlap because the unwinder never
> has to return through any of the jumped-to call frames, and thus
> only a minimum amount of saved data need be stored, and for example,
> need not include the process signal mask information. This is all
> an optimization to reduce stack usage when pushing cancellation
> handlers.
> ~~~

Will fix it.

>>    unwind_buf.priv.data.prev = NULL;
>>    unwind_buf.priv.data.cleanup = NULL;
>>
>> -  int not_first_call;
>> -  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
>
> OK.
>
>>    if (__glibc_likely (! not_first_call))
>>      {
>>        /* Store the new cleanup handler info.  */
>> diff --git a/setjmp/longjmp.c b/setjmp/longjmp.c
>> index a2a7065a85..453889e103 100644
>> --- a/setjmp/longjmp.c
>> +++ b/setjmp/longjmp.c
>> @@ -40,9 +40,11 @@ __libc_siglongjmp (sigjmp_buf env, int val)
>>  }
>>
>>  #ifndef __libc_siglongjmp
>> +# ifndef __libc_longjmp
>>  /* __libc_longjmp is a private interface for cancellation implementation
>>     in libpthread.  */
>>  strong_alias (__libc_siglongjmp, __libc_longjmp)
>> +# endif
>
> OK.
>
>>  weak_alias (__libc_siglongjmp, _longjmp)
>>  weak_alias (__libc_siglongjmp, longjmp)
>>  weak_alias (__libc_siglongjmp, siglongjmp)
>> diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
>> index c0ed767a0d..90a6bbcf32 100644
>> --- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
>> +++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
>> @@ -22,8 +22,8 @@
>>  #include <bits/types/__sigset_t.h>
>>
>>  /* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
>> -   Define it to 513 to leave some rooms for future use.  */
>> -#define _JUMP_BUF_SIGSET_NSIG        513
>> +   Define it to 97 to leave some rooms for future use.  */
>
> OK.
>
>> +#define _JUMP_BUF_SIGSET_NSIG        97
>
> Please provide proof in the way of a comment or rewriting this constant
> to show that it places the shadow stack pointer on both x86_64 and x32
> into the range of the private pad.

sysdeps/x86/nptl/pt-longjmp.c has

_Static_assert ((sizeof (struct pthread_unwind_buf)
>= (SHADOW_STACK_POINTER_OFFSET
     + SHADOW_STACK_POINTER_SIZE)),
"size of struct pthread_unwind_buf < "
"(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");

If shadow stack pointer is saved in the offset bigger than the size of
struct pthread_unwind_buf, assert will trigger at compile-time.

> Also, from commit  f33632ccd1dec3217583fcfdd965afb62954203c,
> where did this math come from?
>
> ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>
> Why the +7?

_JUMP_BUF_SIGSET_NSIG is the biggest signal number + 1.
_JUMP_BUF_SIGSET_NSIG - 1 gives the biggest signal number.
_JUMP_BUF_SIGSET_NSIG - 1 + 7 rounds up to the number of bytes
which are needed to store the biggest signal number.

>>  /* Number of longs to hold all signals.  */
>>  #define _JUMP_BUF_SIGSET_NWORDS \
>>    ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
>> index 0d0326c21a..d25d6f0ae4 100644
>> --- a/sysdeps/x86/Makefile
>> +++ b/sysdeps/x86/Makefile
>> @@ -8,3 +8,7 @@ sysdep-dl-routines += dl-get-cpu-features
>>  tests += tst-get-cpu-features
>>  tests-static += tst-get-cpu-features-static
>>  endif
>> +
>> +ifeq ($(subdir),setjmp)
>> +sysdep_routines += __longjmp_cancel
>> +endif
>
> OK.
>
>> diff --git a/sysdeps/x86/__longjmp_cancel.S b/sysdeps/x86/__longjmp_cancel.S
>> new file mode 100644
>> index 0000000000..b57dbfa376
>> --- /dev/null
>> +++ b/sysdeps/x86/__longjmp_cancel.S
>> @@ -0,0 +1,20 @@
>> +/* __longjmp_cancel for x86.
>> +   Copyright (C) 2018 Free Software Foundation, Inc.
>> +   This file is part of the GNU C Library.
>> +
>> +   The GNU C Library is free software; you can redistribute it and/or
>> +   modify it under the terms of the GNU Lesser General Public
>> +   License as published by the Free Software Foundation; either
>> +   version 2.1 of the License, or (at your option) any later version.
>> +
>> +   The GNU C Library is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> +   Lesser General Public License for more details.
>> +
>> +   You should have received a copy of the GNU Lesser General Public
>> +   License along with the GNU C Library; if not, see
>> +   <http://www.gnu.org/licenses/>.  */
>> +
>> +#define __longjmp __longjmp_cancel
>> +#include <__longjmp.S>
>
> OK.
>
>> diff --git a/sysdeps/x86/longjmp.c b/sysdeps/x86/longjmp.c
>> new file mode 100644
>> index 0000000000..a53f31e1dd
>> --- /dev/null
>> +++ b/sysdeps/x86/longjmp.c
>> @@ -0,0 +1,45 @@
>> +/* __libc_siglongjmp for x86.
>> +   Copyright (C) 2018 Free Software Foundation, Inc.
>> +   This file is part of the GNU C Library.
>> +
>> +   The GNU C Library is free software; you can redistribute it and/or
>> +   modify it under the terms of the GNU Lesser General Public
>> +   License as published by the Free Software Foundation; either
>> +   version 2.1 of the License, or (at your option) any later version.
>> +
>> +   The GNU C Library is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> +   Lesser General Public License for more details.
>> +
>> +   You should have received a copy of the GNU Lesser General Public
>> +   License along with the GNU C Library; if not, see
>> +   <http://www.gnu.org/licenses/>.  */
>> +
>> +#define __libc_longjmp __redirect___libc_longjmp
>> +#include <setjmp/longjmp.c>
>> +#undef __libc_longjmp
>> +
>> +extern void __longjmp_cancel (__jmp_buf __env, int __val)
>> +     __attribute__ ((__noreturn__)) attribute_hidden;
>> +
>> +/* Since __libc_longjmp is a private interface for cancellation
>> +   implementation in libpthread, there is no need to restore shadow
>> +   stack register.  */
>> +
>> +void
>> +__libc_longjmp (sigjmp_buf env, int val)
>> +{
>> +  /* Perform any cleanups needed by the frames being unwound.  */
>> +  _longjmp_unwind (env, val);
>
> OK.
>
>> +
>> +  if (env[0].__mask_was_saved)
>> +    /* Restore the saved signal mask.  */
>> +    (void) __sigprocmask (SIG_SETMASK,
>> +                       (sigset_t *) &env[0].__saved_mask,
>> +                       (sigset_t *) NULL);
>
> OK.
>
>> +
>> +  /* Call the machine-dependent function to restore machine state
>> +     without shadow stack.  */
>> +  __longjmp_cancel (env[0].__jmpbuf, val ?: 1);
>
> OK.
>
>> +}
>> diff --git a/sysdeps/x86/nptl/pt-longjmp.c b/sysdeps/x86/nptl/pt-longjmp.c
>> new file mode 100644
>> index 0000000000..7eb8651cfe
>> --- /dev/null
>> +++ b/sysdeps/x86/nptl/pt-longjmp.c
>> @@ -0,0 +1,97 @@
>> +/* ABI compatibility for 'longjmp' and 'siglongjmp' symbols in libpthread ABI.
>> +   X86 version.
>> +   Copyright (C) 18 Free Software Foundation, Inc.
>> +   This file is part of the GNU C Library.
>> +
>> +   The GNU C Library is free software; you can redistribute it and/or
>> +   modify it under the terms of the GNU Lesser General Public
>> +   License as published by the Free Software Foundation; either
>> +   version 2.1 of the License, or (at your option) any later version.
>> +
>> +   The GNU C Library is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>> +   Lesser General Public License for more details.
>> +
>> +   You should have received a copy of the GNU Lesser General Public
>> +   License along with the GNU C Library; if not, see
>> +   <http://www.gnu.org/licenses/>.  */
>> +
>> +/* <nptl/descr.h> has
>> +
>> +struct pthread_unwind_buf
>> +{
>> +  struct
>> +  {
>> +    __jmp_buf jmp_buf;
>> +    int mask_was_saved;
>> +  } cancel_jmp_buf[1];
>> +
>> +  union
>> +  {
>> +    void *pad[4];
>> +    struct
>> +    {
>> +      struct pthread_unwind_buf *prev;
>> +      struct _pthread_cleanup_buffer *cleanup;
>> +      int canceltype;
>> +    } data;
>> +  } priv;
>> +};
>> +
>> +  The pad array in struct pthread_unwind_buf is used by setjmp to save
>
> This appears to be SHADOW_STACK_POINTER_OFFSET in subsequent patches.
>
> However on your hjl/cet/master branch it appears that this offset is not
> defined to be *just after* the mask_was_saved?

sysdeps/unix/sysv/linux/x86/setjmpP.h has

typedef struct
  {
    unsigned long int __val[_JUMP_BUF_SIGSET_NWORDS];
  } __jmp_buf_sigset_t;

typedef union
  {
    __sigset_t __saved_mask_compat;
    struct
      {
         __jmp_buf_sigset_t __saved_mask;
          /* Used for shadow stack pointer.  */
         unsigned long int __shadow_stack_pointer;
      } __saved;
  } __jmpbuf_arch_t;

__shadow_stack_pointer is placed after __saved_mask, aka mask_was_saved.

>> +  shadow stack register.  Assert that size of struct pthread_unwind_buf
>> +  is no less than offset of shadow stack pointer plus shadow stack
>> +  pointer size.
>> +
>> +  NB: setjmp is called in libpthread to save shadow stack register.  But
>> +  __libc_unwind_longjmp doesn't restore shadow stack register since they
>> +  never return after longjmp.  */
>> +
>> +#include <pthreadP.h>
>> +#include <jmp_buf-ssp.h>
>> +
>> +#ifdef __x86_64__
>> +# define SHADOW_STACK_POINTER_SIZE 8
>> +#else
>> +# define SHADOW_STACK_POINTER_SIZE 4
>> +#endif
>> +
>> +_Static_assert ((sizeof (struct pthread_unwind_buf)
>> +              >= (SHADOW_STACK_POINTER_OFFSET
>> +                  + SHADOW_STACK_POINTER_SIZE)),
>> +             "size of struct pthread_unwind_buf < "
>> +             "(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");
>
> OK.
>
>> +
>> +#include <shlib-compat.h>
>> +
>> +/* libpthread once had its own longjmp (and siglongjmp alias), though there
>> +   was no apparent reason for it.  There is no use in having a separate
>> +   symbol in libpthread, but the historical ABI requires it.  For static
>> +   linking, there is no need to provide anything here--the libc version
>> +   will be linked in.  For shared library ABI compatibility, there must be
>> +   longjmp and siglongjmp symbols in libpthread.so.
>> +
>> +   With an IFUNC resolver, it would be possible to avoid the indirection,
>> +   but the IFUNC resolver might run before the __libc_longjmp symbol has
>> +   been relocated, in which case the IFUNC resolver would not be able to
>> +   provide the correct address.  */
>> +
>> +#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_22)
>> +
>> +static void __attribute__ ((noreturn, used))
>> +longjmp_compat (jmp_buf env, int val)
>> +{
>> +  /* NB: We call __libc_siglongjmp,  instead of __libc_longjmp, since
>> +     __libc_longjmp is a private interface for cancellation which
>> +     doesn't restore shadow stack register.  */
>> +  __libc_siglongjmp (env, val);
>
> OK.
>
>> +}
>> +
>> +strong_alias (longjmp_compat, longjmp_alias)
>> +compat_symbol (libpthread, longjmp_alias, longjmp, GLIBC_2_0);
>> +
>> +strong_alias (longjmp_alias, siglongjmp_alias)
>> +compat_symbol (libpthread, siglongjmp_alias, siglongjmp, GLIBC_2_0);
>> +
>> +#endif
>> -- 2.14.3
>
>
> --
> Cheers,
> Carlos.



-- 
H.J.