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 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.

Thanks,
Yu-cheng