1// SPDX-License-Identifier: GPL-2.0
2
3#include <linux/ptrace.h>
4#include <asm/bugs.h>
5#include <asm/msr.h>
6#include <asm/traps.h>
7
8enum cp_error_code {
9 CP_EC = (1 << 15) - 1,
10
11 CP_RET = 1,
12 CP_IRET = 2,
13 CP_ENDBR = 3,
14 CP_RSTRORSSP = 4,
15 CP_SETSSBSY = 5,
16
17 CP_ENCL = 1 << 15,
18};
19
20static const char cp_err[][10] = {
21 [0] = "unknown",
22 [1] = "near ret",
23 [2] = "far/iret",
24 [3] = "endbranch",
25 [4] = "rstorssp",
26 [5] = "setssbsy",
27};
28
29static const char *cp_err_string(unsigned long error_code)
30{
31 unsigned int cpec = error_code & CP_EC;
32
33 if (cpec >= ARRAY_SIZE(cp_err))
34 cpec = 0;
35 return cp_err[cpec];
36}
37
38static void do_unexpected_cp(struct pt_regs *regs, unsigned long error_code)
39{
40 WARN_ONCE(1, "Unexpected %s #CP, error_code: %s\n",
41 user_mode(regs) ? "user mode" : "kernel mode",
42 cp_err_string(error_code));
43}
44
45static DEFINE_RATELIMIT_STATE(cpf_rate, DEFAULT_RATELIMIT_INTERVAL,
46 DEFAULT_RATELIMIT_BURST);
47
48static void do_user_cp_fault(struct pt_regs *regs, unsigned long error_code)
49{
50 struct task_struct *tsk;
51 unsigned long ssp;
52
53 /*
54 * An exception was just taken from userspace. Since interrupts are disabled
55 * here, no scheduling should have messed with the registers yet and they
56 * will be whatever is live in userspace. So read the SSP before enabling
57 * interrupts so locking the fpregs to do it later is not required.
58 */
59 rdmsrq(MSR_IA32_PL3_SSP, ssp);
60
61 cond_local_irq_enable(regs);
62
63 tsk = current;
64 tsk->thread.error_code = error_code;
65 tsk->thread.trap_nr = X86_TRAP_CP;
66
67 /* Ratelimit to prevent log spamming. */
68 if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
69 __ratelimit(&cpf_rate)) {
70 pr_emerg("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)%s",
71 tsk->comm, task_pid_nr(tsk),
72 regs->ip, regs->sp, ssp, error_code,
73 cp_err_string(error_code),
74 error_code & CP_ENCL ? " in enclave" : "");
75 print_vma_addr(KERN_CONT " in ", rip: regs->ip);
76 pr_cont("\n");
77 }
78
79 force_sig_fault(SIGSEGV, SEGV_CPERR, addr: (void __user *)0);
80 cond_local_irq_disable(regs);
81}
82
83static __ro_after_init bool ibt_fatal = true;
84
85/*
86 * By definition, all missing-ENDBRANCH #CPs are a result of WFE && !ENDBR.
87 *
88 * For the kernel IBT no ENDBR selftest where #CPs are deliberately triggered,
89 * the WFE state of the interrupted context needs to be cleared to let execution
90 * continue. Otherwise when the CPU resumes from the instruction that just
91 * caused the previous #CP, another missing-ENDBRANCH #CP is raised and the CPU
92 * enters a dead loop.
93 *
94 * This is not a problem with IDT because it doesn't preserve WFE and IRET doesn't
95 * set WFE. But FRED provides space on the entry stack (in an expanded CS area)
96 * to save and restore the WFE state, thus the WFE state is no longer clobbered,
97 * so software must clear it.
98 */
99static void ibt_clear_fred_wfe(struct pt_regs *regs)
100{
101 /*
102 * No need to do any FRED checks.
103 *
104 * For IDT event delivery, the high-order 48 bits of CS are pushed
105 * as 0s into the stack, and later IRET ignores these bits.
106 *
107 * For FRED, a test to check if fred_cs.wfe is set would be dropped
108 * by compilers.
109 */
110 regs->fred_cs.wfe = 0;
111}
112
113static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
114{
115 if ((error_code & CP_EC) != CP_ENDBR) {
116 do_unexpected_cp(regs, error_code);
117 return;
118 }
119
120 if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
121 regs->ax = 0;
122 ibt_clear_fred_wfe(regs);
123 return;
124 }
125
126 pr_err("Missing ENDBR: %pS\n", (void *)instruction_pointer(regs));
127 if (!ibt_fatal) {
128 printk(KERN_DEFAULT CUT_HERE);
129 __warn(__FILE__, __LINE__, caller: (void *)regs->ip, TAINT_WARN, regs, NULL);
130 ibt_clear_fred_wfe(regs);
131 return;
132 }
133 BUG();
134}
135
136static int __init ibt_setup(char *str)
137{
138 if (!strcmp(str, "off"))
139 setup_clear_cpu_cap(X86_FEATURE_IBT);
140
141 if (!strcmp(str, "warn"))
142 ibt_fatal = false;
143
144 return 1;
145}
146
147__setup("ibt=", ibt_setup);
148
149DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
150{
151 if (user_mode(regs)) {
152 if (cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
153 do_user_cp_fault(regs, error_code);
154 else
155 do_unexpected_cp(regs, error_code);
156 } else {
157 if (cpu_feature_enabled(X86_FEATURE_IBT))
158 do_kernel_cp_fault(regs, error_code);
159 else
160 do_unexpected_cp(regs, error_code);
161 }
162}
163

source code of linux/arch/x86/kernel/cet.c