Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)

Yu, Yu-cheng
2021-05-06T22:05:27Z

On 5/4/2021 1:49 PM, Yu, Yu-cheng wrote:
> On 4/30/2021 11:32 AM, Yu, Yu-cheng wrote:
>> On 4/30/2021 10:47 AM, Andy Lutomirski wrote:
>>> On Fri, Apr 30, 2021 at 10:00 AM Yu, Yu-cheng <yu-cheng.yu@intel.com>
>>> wrote:
>>>>
>>>> On 4/28/2021 4:03 PM, Andy Lutomirski wrote:
>>>>> On Tue, Apr 27, 2021 at 1:44 PM Yu-cheng Yu <yu-cheng.yu@intel.com>
>>>>> wrote:
>>>>>>
>>>>>> When shadow stack is enabled, a task's shadow stack states must be
>>>>>> saved
>>>>>> along with the signal context and later restored in sigreturn.
>>>>>> However,
>>>>>> currently there is no systematic facility for extending a signal
>>>>>> context.
>>>>>> There is some space left in the ucontext, but changing ucontext is
>>>>>> likely
>>>>>> to create compatibility issues and there is not enough space for
>>>>>> further
>>>>>> extensions.
>>>>>>
>>>>>> Introduce a signal context extension struct 'sc_ext', which is
>>>>>> used to save
>>>>>> shadow stack restore token address.  The extension is located
>>>>>> above the fpu
>>>>>> states, plus alignment.  The struct can be extended (such as the
>>>>>> ibt's
>>>>>> wait_endbr status to be introduced later), and sc_ext.total_size
>>>>>> field
>>>>>> keeps track of total size.
>>>>>
>>>>> I still don't like this.
>>>>>

[...]

>>>>>
>>>>> That's where we are right now upstream.  The kernel has a parser for
>>>>> the FPU state that is bugs piled upon bugs and is going to have to be
>>>>> rewritten sometime soon.  On top of all this, we have two upcoming
>>>>> features, both of which require different kinds of extensions:
>>>>>
>>>>> 1. AVX-512.  (Yeah, you thought this story was over a few years ago,
>>>>> but no.  And AMX makes it worse.)  To make a long story short, we
>>>>> promised user code many years ago that a signal frame fit in 2048
>>>>> bytes with some room to spare.  With AVX-512 this is false.  With AMX
>>>>> it's so wrong it's not even funny.  The only way out of the mess
>>>>> anyone has come up with involves making the length of the FPU state
>>>>> vary depending on which features are INIT, i.e. making it more compact
>>>>> than "compact" mode is.  This has a side effect: it's no longer
>>>>> possible to modify the state in place, because enabling a feature with
>>>>> no space allocated will make the structure bigger, and the stack won't
>>>>> have room.  Fortunately, one can relocate the entire FPU state, update
>>>>> the pointer in mcontext, and the kernel will happily follow the
>>>>> pointer.  So new code on a new kernel using a super-compact state
>>>>> could expand the state by allocating new memory (on the heap? very
>>>>> awkwardly on the stack?) and changing the pointer.  For all we know,
>>>>> some code already fiddles with the pointer.  This is great, except
>>>>> that your patch sticks more data at the end of the FPU block that no
>>>>> one is expecting, and your sigreturn code follows that pointer, and
>>>>> will read off into lala land.
>>>>>
>>>>
>>>> Then, what about we don't do that at all.  Is it possible from now
>>>> on we
>>>> don't stick more data at the end, and take the relocating-fpu approach?
>>>>
>>>>> 2. CET.  CET wants us to find a few more bytes somewhere, and those
>>>>> bytes logically belong in ucontext, and here we are.
>>>>>
>>>>
>>>> Fortunately, we can spare CET the need of ucontext extension.  When the
>>>> kernel handles sigreturn, the user-mode shadow stack pointer is
>>>> right at
>>>> the restore token.  There is no need to put that in ucontext.
>>>
>>> That seems entirely reasonable.  This might also avoid needing to
>>> teach CRIU about CET at all.
>>>
>>>>
>>>> However, the WAIT_ENDBR status needs to be saved/restored for signals.
>>>> Since IBT is now dependent on shadow stack, we can use a spare bit of
>>>> the shadow stack restore token for that.
>>>
>>> That seems like unnecessary ABI coupling.  We have plenty of bits in
>>> uc_flags, and we have an entire reserved word in sigcontext.  How
>>> about just sticking this bit in one of those places?
>>
>> Yes, I will make it UC_WAIT_ENDBR.
>
> Personally, I think an explicit flag is cleaner than using a reserved
> word somewhere.  However, there is a small issue: ia32 has no uc_flags.
>
> This series can support legacy apps up to now.  But, instead of creating
> too many special cases, perhaps we should drop CET support of ia32?
>
> Thoughts?
>

Once we have UC_WAIT_ENDBR, IBT signal handling becomes quite simple.
Like the following:

diff --git a/arch/x86/include/uapi/asm/ucontext.h
b/arch/x86/include/uapi/asm/ucontext.h
index 5657b7a49f03..96375d609e11 100644
--- a/arch/x86/include/uapi/asm/ucontext.h
+++ b/arch/x86/include/uapi/asm/ucontext.h
@@ -49,6 +49,11 @@
*/
#define UC_SIGCONTEXT_SS 0x2
#define UC_STRICT_RESTORE_SS 0x4
+
+/*
+ * UC_WAIT_ENDBR indicates the task is in wait-ENDBR status.
+ */
+#define UC_WAIT_ENDBR 0x08
#endif

#include <asm-generic/ucontext.h>
diff --git a/arch/x86/kernel/ibt.c b/arch/x86/kernel/ibt.c
index d2563dd4759f..da804314ddc4 100644
--- a/arch/x86/kernel/ibt.c
+++ b/arch/x86/kernel/ibt.c
@@ -66,3 +66,32 @@ void ibt_disable(void)
ibt_set_clear_msr_bits(0, CET_ENDBR_EN);
current->thread.cet.ibt = 0;
}
+
+int ibt_get_clear_wait_endbr(void)
+{
+ u64 msr_val = 0;
+
+ if (!current->thread.cet.ibt)
+ return 0;
+
+ fpregs_lock();
+
+ if (test_thread_flag(TIF_NEED_FPU_LOAD))
+ __fpregs_load_activate();
+
+ if (!rdmsrl_safe(MSR_IA32_U_CET, &msr_val))
+ wrmsrl(MSR_IA32_U_CET, msr_val & ~CET_WAIT_ENDBR);
+
+ fpregs_unlock();
+
+ return msr_val & CET_WAIT_ENDBR;
+}
+
+int ibt_set_wait_endbr(void)
+{
+ if (!current->thread.cet.ibt)
+ return 0;
+
+
+ return ibt_set_clear_msr_bits(CET_WAIT_ENDBR, 0);
+}
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 66b662e57e19..5afd15419006 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -46,6 +46,7 @@
#include <asm/syscall.h>
#include <asm/sigframe.h>
#include <asm/signal.h>
+#include <asm/cet.h>

#ifdef CONFIG_X86_64
/*
@@ -134,6 +135,9 @@ static int restore_sigcontext(struct pt_regs *regs,
*/
if (unlikely(!(uc_flags & UC_STRICT_RESTORE_SS) &&
user_64bit_mode(regs)))
force_valid_ss(regs);
+
+ if (uc_flags & UC_WAIT_ENDBR)
+ ibt_set_wait_endbr();
#endif

return fpu__restore_sig((void __user *)sc.fpstate,
@@ -433,6 +437,9 @@ static unsigned long frame_uc_flags(struct pt_regs
*regs)
if (likely(user_64bit_mode(regs)))
flags |= UC_STRICT_RESTORE_SS;

+ if (ibt_get_clear_wait_endbr())
+ flags |= UC_WAIT_ENDBR;
+
return flags;
}

However, this cannot handle ia32 with no SA_SIGINFO. For that, can we
create a synthetic token on the shadow stack?

- The token points to itself with reserved bit[1] set, and cannot be
used for RSTORSSP.
- The token only exists for ia32 with no SA_SIGINFO *AND* when the task
is in wait-endbr.

The signal shadow stack will look like this:

--> ssp before signal
synthetic IBT token (for ia32 no SA_SIGINFO)
shadow stack restore token
sigreturn address

The synthetic token is not valid in other situations.
How is that?

Thanks,
Yu-cheng