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

[Yu, Yu-cheng]

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@xxxxxxxxx> 
> > > 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@xxxxxxxxx> 
> > > > > 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