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

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

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.
>>>>
>>>> Here's how the signal layout works, for better or for worse:
>>>>
>>>> The kernel has:
>>>>
>>>> struct rt_sigframe {
>>>>       char __user *pretcode;
>>>>       struct ucontext uc;
>>>>       struct siginfo info;
>>>>       /* fp state follows here */
>>>> };
>>>>
>>>> This is roughly the actual signal frame.  But userspace does not have
>>>> this struct declared, and user code does not know the sizes of the
>>>> fields.  So it's accessed in a nonsensical way.  The signal handler
>>>> function is passed a pointer to the whole sigframe implicitly in RSP,
>>>> a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX.
>>>> User code can *find* the fp state by following a pointer from
>>>> mcontext, which is, in turn, found via uc:
>>>>
>>>> struct ucontext {
>>>>       unsigned long      uc_flags;
>>>>       struct ucontext  *uc_link;
>>>>       stack_t          uc_stack;
>>>>       struct sigcontext uc_mcontext;  <-- fp pointer is in here
>>>>       sigset_t      uc_sigmask;    /* mask last for extensibility */
>>>> };
>>>>
>>>> The kernel, in sigreturn, works a bit differently.  The sigreturn
>>>> variants know the base address of the frame but don't have the benefit
>>>> of receiving pointers to the fields.  So instead the kernel takes
>>>> advantage of the fact that it knows the offset to uc and parses uc
>>>> accordingly.  And the kernel follows the pointer in mcontext to find
>>>> the fp state.  The latter bit is quite important later.  The kernel
>>>> does not parse info at all.
>>>>
>>>> The fp state is its own mess.  When XSAVE happened, Intel kindly (?)
>>>> gave us a software defined area between the "legacy" x87 region and
>>>> the modern supposedly extensible part.  Linux sticks the following
>>>> structure in that hole:
>>>>
>>>> struct _fpx_sw_bytes {
>>>>       /*
>>>>        * If set to FP_XSTATE_MAGIC1 then this is an xstate context.
>>>>        * 0 if a legacy frame.
>>>>        */
>>>>       __u32                magic1;
>>>>
>>>>       /*
>>>>        * Total size of the fpstate area:
>>>>        *
>>>>        *  - if magic1 == 0 then it's sizeof(struct _fpstate)
>>>>        *  - if magic1 == FP_XSTATE_MAGIC1 then it's sizeof(struct 
>>>> _xstate)
>>>>        *    plus extensions (if any)
>>>>        */
>>>>       __u32                extended_size;
>>>>
>>>>       /*
>>>>        * Feature bit mask (including FP/SSE/extended state) that is 
>>>> present
>>>>        * in the memory layout:
>>>>        */
>>>>       __u64                xfeatures;
>>>>
>>>>       /*
>>>>        * Actual XSAVE state size, based on the xfeatures saved in 
>>>> the layout.
>>>>        * 'extended_size' is greater than 'xstate_size':
>>>>        */
>>>>       __u32                xstate_size;
>>>>
>>>>       /* For future use: */
>>>>       __u32                padding[7];
>>>> };
>>>>
>>>>
>>>> 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?

Thanks,
Yu-cheng