[PATCH 06/35] x86/cet: Add control-protection fault handler

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

From: Yu-cheng Yu <yu-cheng.yu@intel.com>

A control-protection fault is triggered when a control-flow transfer
attempt violates Shadow Stack or Indirect Branch Tracking constraints.
For example, the return address for a RET instruction differs from the copy
on the shadow stack; or an indirect JMP instruction, without the NOTRACK
prefix, arrives at a non-ENDBR opcode.

The control-protection fault handler works in a similar way as the general
protection fault handler.  It provides the si_code SEGV_CPERR to the signal
handler.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
---

v1:
 - Update static asserts for NSIGSEGV

Yu-cheng v29:
 - Remove pr_emerg() since it is followed by die().
 - Change boot_cpu_has() to cpu_feature_enabled().

Yu-cheng v25:
 - Change CONFIG_X86_CET to CONFIG_X86_SHADOW_STACK.
 - Change X86_FEATURE_CET to X86_FEATURE_SHSTK.
 
 arch/arm/kernel/signal.c           |  2 +-
 arch/arm64/kernel/signal.c         |  2 +-
 arch/arm64/kernel/signal32.c       |  2 +-
 arch/sparc/kernel/signal32.c       |  2 +-
 arch/sparc/kernel/signal_64.c      |  2 +-
 arch/x86/include/asm/idtentry.h    |  4 ++
 arch/x86/kernel/idt.c              |  4 ++
 arch/x86/kernel/signal_compat.c    |  2 +-
 arch/x86/kernel/traps.c            | 62 ++++++++++++++++++++++++++++++
 include/uapi/asm-generic/siginfo.h |  3 +-
 10 files changed, 78 insertions(+), 7 deletions(-)

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index c532a6041066..59aaadce9d52 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -681,7 +681,7 @@ asmlinkage void do_rseq_syscall(struct pt_regs *regs)
  */
 static_assert(NSIGILL	== 11);
 static_assert(NSIGFPE	== 15);
-static_assert(NSIGSEGV	== 9);
+static_assert(NSIGSEGV	== 10);
 static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index d8aaf4b6f432..d2da57c415b8 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -983,7 +983,7 @@ void __init minsigstksz_setup(void)
  */
 static_assert(NSIGILL	== 11);
 static_assert(NSIGFPE	== 15);
-static_assert(NSIGSEGV	== 9);
+static_assert(NSIGSEGV	== 10);
 static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index d984282b979f..8776a34c6444 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -460,7 +460,7 @@ void compat_setup_restart_syscall(struct pt_regs *regs)
  */
 static_assert(NSIGILL	== 11);
 static_assert(NSIGFPE	== 15);
-static_assert(NSIGSEGV	== 9);
+static_assert(NSIGSEGV	== 10);
 static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
index 6cc124a3bb98..dc50b2a78692 100644
--- a/arch/sparc/kernel/signal32.c
+++ b/arch/sparc/kernel/signal32.c
@@ -752,7 +752,7 @@ asmlinkage int do_sys32_sigstack(u32 u_ssptr, u32 u_ossptr, unsigned long sp)
  */
 static_assert(NSIGILL	== 11);
 static_assert(NSIGFPE	== 15);
-static_assert(NSIGSEGV	== 9);
+static_assert(NSIGSEGV	== 10);
 static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
index 2a78d2af1265..7fe2bd37bd1a 100644
--- a/arch/sparc/kernel/signal_64.c
+++ b/arch/sparc/kernel/signal_64.c
@@ -562,7 +562,7 @@ void do_notify_resume(struct pt_regs *regs, unsigned long orig_i0, unsigned long
  */
 static_assert(NSIGILL	== 11);
 static_assert(NSIGFPE	== 15);
-static_assert(NSIGSEGV	== 9);
+static_assert(NSIGSEGV	== 10);
 static_assert(NSIGBUS	== 5);
 static_assert(NSIGTRAP	== 6);
 static_assert(NSIGCHLD	== 6);
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 1345088e9902..a90791433152 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -562,6 +562,10 @@ DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_SS,	exc_stack_segment);
 DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_GP,	exc_general_protection);
 DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_AC,	exc_alignment_check);
 
+#ifdef CONFIG_X86_SHADOW_STACK
+DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_CP, exc_control_protection);
+#endif
+
 /* Raw exception entries which need extra work */
 DECLARE_IDTENTRY_RAW(X86_TRAP_UD,		exc_invalid_op);
 DECLARE_IDTENTRY_RAW(X86_TRAP_BP,		exc_int3);
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index df0fa695bb09..9f1bdaabc246 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -113,6 +113,10 @@ static const __initconst struct idt_data def_idts[] = {
 #elif defined(CONFIG_X86_32)
 	SYSG(IA32_SYSCALL_VECTOR,	entry_INT80_32),
 #endif
+
+#ifdef CONFIG_X86_SHADOW_STACK
+	INTG(X86_TRAP_CP,		asm_exc_control_protection),
+#endif
 };
 
 /*
diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index b52407c56000..ff50cd978ea5 100644
--- a/arch/x86/kernel/signal_compat.c
+++ b/arch/x86/kernel/signal_compat.c
@@ -27,7 +27,7 @@ static inline void signal_compat_build_tests(void)
 	 */
 	BUILD_BUG_ON(NSIGILL  != 11);
 	BUILD_BUG_ON(NSIGFPE  != 15);
-	BUILD_BUG_ON(NSIGSEGV != 9);
+	BUILD_BUG_ON(NSIGSEGV != 10);
 	BUILD_BUG_ON(NSIGBUS  != 5);
 	BUILD_BUG_ON(NSIGTRAP != 6);
 	BUILD_BUG_ON(NSIGCHLD != 6);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c9d566dcf89a..54b7a146fd5e 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -39,6 +39,7 @@
 #include <linux/io.h>
 #include <linux/hardirq.h>
 #include <linux/atomic.h>
+#include <linux/nospec.h>
 
 #include <asm/stacktrace.h>
 #include <asm/processor.h>
@@ -641,6 +642,67 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
 	cond_local_irq_disable(regs);
 }
 
+#ifdef CONFIG_X86_SHADOW_STACK
+static const char * const control_protection_err[] = {
+	"unknown",
+	"near-ret",
+	"far-ret/iret",
+	"endbranch",
+	"rstorssp",
+	"setssbsy",
+	"unknown",
+};
+
+static DEFINE_RATELIMIT_STATE(cpf_rate, DEFAULT_RATELIMIT_INTERVAL,
+			      DEFAULT_RATELIMIT_BURST);
+
+/*
+ * When a control protection exception occurs, send a signal to the responsible
+ * application.  Currently, control protection is only enabled for user mode.
+ * This exception should not come from kernel mode.
+ */
+DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
+{
+	struct task_struct *tsk;
+
+	if (!user_mode(regs)) {
+		die("kernel control protection fault", regs, error_code);
+		panic("Unexpected kernel control protection fault.  Machine halted.");
+	}
+
+	cond_local_irq_enable(regs);
+
+	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+		WARN_ONCE(1, "Control protection fault with CET support disabled\n");
+
+	tsk = current;
+	tsk->thread.error_code = error_code;
+	tsk->thread.trap_nr = X86_TRAP_CP;
+
+	/*
+	 * Ratelimit to prevent log spamming.
+	 */
+	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+	    __ratelimit(&cpf_rate)) {
+		unsigned long ssp;
+		int cpf_type;
+
+		cpf_type = array_index_nospec(error_code, ARRAY_SIZE(control_protection_err));
+
+		rdmsrl(MSR_IA32_PL3_SSP, ssp);
+		pr_emerg("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)",
+			 tsk->comm, task_pid_nr(tsk),
+			 regs->ip, regs->sp, ssp, error_code,
+			 control_protection_err[cpf_type]);
+		print_vma_addr(KERN_CONT " in ", regs->ip);
+		pr_cont("\n");
+	}
+
+	force_sig_fault(SIGSEGV, SEGV_CPERR, (void __user *)0);
+	cond_local_irq_disable(regs);
+}
+#endif
+
 static bool do_int3(struct pt_regs *regs)
 {
 	int res;
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index 3ba180f550d7..081f4b37d22c 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -240,7 +240,8 @@ typedef struct siginfo {
 #define SEGV_ADIPERR	7	/* Precise MCD exception */
 #define SEGV_MTEAERR	8	/* Asynchronous ARM MTE error */
 #define SEGV_MTESERR	9	/* Synchronous ARM MTE exception */
-#define NSIGSEGV	9
+#define SEGV_CPERR	10	/* Control protection fault */
+#define NSIGSEGV	10
 
 /*
  * SIGBUS si_codes
-- 
2.17.1