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

Yu, Yu-cheng
2021-04-30T17:01:00Z

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.

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.

I have tested the change, and will send out another version of the whole
series.

> This is *almost*, but not quite, easy: struct ucontext is already
> variable length! Unfortunately, the whole variable length portion is
> used up by uc_sigmask. So I propose that we introduce a brand new
> bona fide extension mechanism. It works like this:
>
> First, we add a struct ucontext_extension at the end. It looks like:
>
> struct ucontext_extension {
> u64 length; /* sizeof(struct ucontext_extension) */
> u64 flags; /* we will want this some day */
> [CET stuff here]
> [future stuff here]
> };
>
> And we locate it by scrounging a word somewhere in ucontext to give
> the offset from the beginning of struct ucontext to
> ucontext_extension. We indicate the presence of this feature using a
> new uc_flags bit. I can think of a couple of vaguely reasonable
> places:
>
> a) the reserved word in sigcontext. This is fine for x86 but not so
> great if other architectures want to do this.
>
> b) uc_link. Fine everywhere but powerpc. Oops.
>
> c) use the high bits of uc_flags. After all, once we add extensions,
> we don't need new flags, so we can steal 16 high bits of uc_flags for
> this.
>
> I think I'm in favor of (c). We do:
>
> (uc_flags & 0xffff0000) == 0: extension not present
>
> Otherwise the extension region is at ucontext + (uc_flags >> 16).
>
> And sigreturn finds the extension the same way, because CRIU can
> already migrate a signal frame from one kernel to another, your patch
> breaks this, and having sigreturn hardcode the offset would also break
> it.
>
> What do you think?
>