[PATCH v26 3/9] x86/cet/ibt: Handle signals for Indirect Branch Tracking

[Yu-cheng Yu]

When an indirect CALL/JMP instruction is executed and before it reaches
the target, it is in 'WAIT_ENDBR' status, which can be read from
MSR_IA32_U_CET.  The status is part of a task's status before a signal is
raised and preserved in the signal frame.  It is restored for sigreturn.

IBT state machine is described in Intel SDM Vol. 1, Sec. 18.3.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---
v25:
- Move the addition of sc_ext.wait_endbr from an earlier shadow stack
  patch to here.
- Change X86_FEATURE_CET to X86_FEATURE_SHSTK.
- Change wrmsrl() to wrmsrl_safe() and handle error.

v24:
- Update for changes from splitting shadow stack and ibt.

 arch/x86/include/uapi/asm/sigcontext.h |  1 +
 arch/x86/kernel/fpu/signal.c           | 33 +++++++++++++++++++++++---
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h
index 10d7fa192d48..ee5bacce7d87 100644
--- a/arch/x86/include/uapi/asm/sigcontext.h
+++ b/arch/x86/include/uapi/asm/sigcontext.h
@@ -203,6 +203,7 @@ struct _xstate {
 struct sc_ext {
 	unsigned long total_size;
 	unsigned long ssp;
+	unsigned long wait_endbr;
 };
 
 /*
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 0488407bec81..0ed01e70b09e 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -71,16 +71,29 @@ int save_extra_state_to_sigframe(int ia32, void __user *fp, void __user *restore
 			return err;
 
 		ext.ssp = token_addr;
+	}
 
+	if (new_ssp || cet->ibt_enabled) {
 		fpregs_lock();
 		if (test_thread_flag(TIF_NEED_FPU_LOAD))
 			__fpregs_load_activate();
 		if (new_ssp)
 			err = wrmsrl_safe(MSR_IA32_PL3_SSP, new_ssp);
+
+		if (!err && cet->ibt_enabled) {
+			u64 msr_val;
+
+			err = rdmsrl_safe(MSR_IA32_U_CET, &msr_val);
+			if (!err && (msr_val & CET_WAIT_ENDBR)) {
+				ext.wait_endbr = 1;
+				msr_val &= ~CET_WAIT_ENDBR;
+				err = wrmsrl_safe(MSR_IA32_U_CET, msr_val);
+			}
+		}
 		fpregs_unlock();
 	}
 
-	if (!err && ext.ssp) {
+	if (!err && (ext.ssp || cet->ibt_enabled)) {
 		void __user *p = fp;
 
 		ext.total_size = sizeof(ext);
@@ -110,7 +123,8 @@ static int get_extra_state_from_sigframe(int ia32, void __user *fp, struct sc_ex
 	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
 		return 0;
 
-	if (!cet->shstk_size)
+	if (!cet->shstk_size &&
+	    !cet->ibt_enabled)
 		return 0;
 
 	memset(ext, 0, sizeof(*ext));
@@ -149,6 +163,19 @@ static int restore_extra_state_to_xregs(struct sc_ext *sc_ext)
 
 	if (cet->shstk_size)
 		err = wrmsrl_safe(MSR_IA32_PL3_SSP, sc_ext->ssp);
+
+	if (err)
+		return err;
+
+	if (cet->ibt_enabled && sc_ext->wait_endbr) {
+		u64 msr_val;
+
+		err = rdmsrl_safe(MSR_IA32_U_CET, &msr_val);
+		if (!err) {
+			msr_val |= CET_WAIT_ENDBR;
+			err = wrmsrl_safe(MSR_IA32_U_CET, msr_val);
+		}
+	}
 #endif
 	return err;
 }
@@ -616,7 +643,7 @@ static unsigned long fpu__alloc_sigcontext_ext(unsigned long sp)
 	 * sigcontext_ext is at: fpu + fpu_user_xstate_size +
 	 * FP_XSTATE_MAGIC2_SIZE, then aligned to 8.
 	 */
-	if (cet->shstk_size)
+	if (cet->shstk_size || cet->ibt_enabled)
 		sp -= (sizeof(struct sc_ext) + 8);
 #endif
 	return sp;
-- 
2.21.0