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Subject[PATCH v10 04/26] x86/cet: Add control-protection fault handler
Date
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>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
v10:
- Change CONFIG_X86_64 to CONFIG_X86_INTEL_CET.

v9:
- Add Shadow Stack pointer to the fault printout.

arch/x86/entry/entry_64.S | 2 +-
arch/x86/include/asm/traps.h | 5 +++
arch/x86/kernel/idt.c | 4 ++
arch/x86/kernel/signal_compat.c | 2 +-
arch/x86/kernel/traps.c | 59 ++++++++++++++++++++++++++++++
include/uapi/asm-generic/siginfo.h | 3 +-
6 files changed, 72 insertions(+), 3 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 0e9504fabe52..f42780922387 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1034,7 +1034,7 @@ idtentry spurious_interrupt_bug do_spurious_interrupt_bug has_error_code=0
idtentry coprocessor_error do_coprocessor_error has_error_code=0
idtentry alignment_check do_alignment_check has_error_code=1
idtentry simd_coprocessor_error do_simd_coprocessor_error has_error_code=0
-
+idtentry control_protection do_control_protection has_error_code=1

/*
* Reload gs selector with exception handling
diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index c26a7e1d8a2c..9bf804709ee6 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -35,6 +35,9 @@ asmlinkage void alignment_check(void);
asmlinkage void machine_check(void);
#endif /* CONFIG_X86_MCE */
asmlinkage void simd_coprocessor_error(void);
+#ifdef CONFIG_X86_INTEL_CET
+asmlinkage void control_protection(void);
+#endif

#if defined(CONFIG_X86_64) && defined(CONFIG_XEN_PV)
asmlinkage void xen_divide_error(void);
@@ -86,6 +89,7 @@ dotraplinkage void do_simd_coprocessor_error(struct pt_regs *regs, long error_co
dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code);
#endif
dotraplinkage void do_mce(struct pt_regs *regs, long error_code);
+dotraplinkage void do_control_protection(struct pt_regs *regs, long error_code);

#ifdef CONFIG_X86_64
asmlinkage __visible notrace struct pt_regs *sync_regs(struct pt_regs *eregs);
@@ -151,6 +155,7 @@ enum {
X86_TRAP_AC, /* 17, Alignment Check */
X86_TRAP_MC, /* 18, Machine Check */
X86_TRAP_XF, /* 19, SIMD Floating-Point Exception */
+ X86_TRAP_CP = 21, /* 21 Control Protection Fault */
X86_TRAP_IRET = 32, /* 32, IRET Exception */
};

diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index 87ef69a72c52..19160c8d734f 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -102,6 +102,10 @@ static const __initconst struct idt_data def_idts[] = {
#elif defined(CONFIG_X86_32)
SYSG(IA32_SYSCALL_VECTOR, entry_INT80_32),
#endif
+
+#ifdef CONFIG_X86_INTEL_CET
+ INTG(X86_TRAP_CP, control_protection),
+#endif
};

/*
diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index 9ccbf0576cd0..c572a3de1037 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 != 7);
+ BUILD_BUG_ON(NSIGSEGV != 8);
BUILD_BUG_ON(NSIGBUS != 5);
BUILD_BUG_ON(NSIGTRAP != 5);
BUILD_BUG_ON(NSIGCHLD != 6);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d54cffdc7cac..d2515dfbc178 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -586,6 +586,65 @@ dotraplinkage void do_general_protection(struct pt_regs *regs, long error_code)
}
NOKPROBE_SYMBOL(do_general_protection);

+static const char * const control_protection_err[] = {
+ "unknown",
+ "near-ret",
+ "far-ret/iret",
+ "endbranch",
+ "rstorssp",
+ "setssbsy",
+};
+
+/*
+ * When a control protection exception occurs, send a signal
+ * to the responsible application. Currently, control
+ * protection is only enabled for the user mode. This
+ * exception should not come from the kernel mode.
+ */
+dotraplinkage void
+do_control_protection(struct pt_regs *regs, long error_code)
+{
+ struct task_struct *tsk;
+
+ RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+ if (notify_die(DIE_TRAP, "control protection fault", regs,
+ error_code, X86_TRAP_CP, SIGSEGV) == NOTIFY_STOP)
+ return;
+ cond_local_irq_enable(regs);
+
+ if (!user_mode(regs))
+ die("kernel control protection fault", regs, error_code);
+
+ if (!static_cpu_has(X86_FEATURE_SHSTK) &&
+ !static_cpu_has(X86_FEATURE_IBT))
+ WARN_ONCE(1, "CET is disabled but got control protection fault\n");
+
+ tsk = current;
+ tsk->thread.error_code = error_code;
+ tsk->thread.trap_nr = X86_TRAP_CP;
+
+ if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+ printk_ratelimit()) {
+ unsigned int max_err;
+ unsigned long ssp;
+
+ max_err = ARRAY_SIZE(control_protection_err) - 1;
+ if ((error_code < 0) || (error_code > max_err))
+ error_code = 0;
+ rdmsrl(MSR_IA32_PL3_SSP, ssp);
+ pr_info("%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[error_code]);
+ print_vma_addr(KERN_CONT " in ", regs->ip);
+ pr_cont("\n");
+ }
+
+ force_sig_fault(SIGSEGV, SEGV_CPERR,
+ (void __user *)uprobe_get_trap_addr(regs));
+}
+NOKPROBE_SYMBOL(do_control_protection);
+
dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
{
if (poke_int3_handler(regs))
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index cb3d6c267181..693071dbe641 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -229,7 +229,8 @@ typedef struct siginfo {
#define SEGV_ACCADI 5 /* ADI not enabled for mapped object */
#define SEGV_ADIDERR 6 /* Disrupting MCD error */
#define SEGV_ADIPERR 7 /* Precise MCD exception */
-#define NSIGSEGV 7
+#define SEGV_CPERR 8
+#define NSIGSEGV 8

/*
* SIGBUS si_codes
--
2.21.0
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