[PATCH v20 3/7] 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>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/x86/kernel/cet.c        | 26 ++++++++++++++++++++++++--
 arch/x86/kernel/fpu/signal.c |  8 +++++---
 2 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index 164139e23fef..ca2dc6b35c4d 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c
@@ -295,6 +295,13 @@ void cet_restore_signal(struct sc_ext *sc_ext)
 		msr_val |= CET_SHSTK_EN;
 	}
 
+	if (cet->ibt_enabled) {
+		msr_val |= (CET_ENDBR_EN | CET_NO_TRACK_EN);
+
+		if (sc_ext->wait_endbr)
+			msr_val |= CET_WAIT_ENDBR;
+	}
+
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		cet_user_state->user_cet = msr_val;
 	else
@@ -335,9 +342,24 @@ int cet_setup_signal(bool ia32, unsigned long rstor_addr, struct sc_ext *sc_ext)
 		sc_ext->ssp = new_ssp;
 	}
 
-	if (ssp) {
+	if (ssp || cet->ibt_enabled) {
 		start_update_msrs();
-		wrmsrl(MSR_IA32_PL3_SSP, ssp);
+
+		if (ssp)
+			wrmsrl(MSR_IA32_PL3_SSP, ssp);
+
+		if (cet->ibt_enabled) {
+			u64 r;
+
+			rdmsrl(MSR_IA32_U_CET, r);
+
+			if (r & CET_WAIT_ENDBR) {
+				sc_ext->wait_endbr = 1;
+				r &= ~CET_WAIT_ENDBR;
+				wrmsrl(MSR_IA32_U_CET, r);
+			}
+		}
+
 		end_update_msrs();
 	}
 
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 270e4649f435..b914d74c8ba6 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -57,7 +57,8 @@ int save_cet_to_sigframe(int ia32, void __user *fp, unsigned long restorer)
 {
 	int err = 0;
 
-	if (!current->thread.cet.shstk_size)
+	if (!current->thread.cet.shstk_size &&
+	    !current->thread.cet.ibt_enabled)
 		return 0;
 
 	if (fp) {
@@ -89,7 +90,8 @@ static int get_cet_from_sigframe(int ia32, void __user *fp, struct sc_ext *ext)
 
 	memset(ext, 0, sizeof(*ext));
 
-	if (!current->thread.cet.shstk_size)
+	if (!current->thread.cet.shstk_size &&
+	    !current->thread.cet.ibt_enabled)
 		return 0;
 
 	if (fp) {
@@ -577,7 +579,7 @@ 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);
 
 	return sp;
-- 
2.21.0