1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * common.c - C code for kernel entry and exit
4  * Copyright (c) 2015 Andrew Lutomirski
5  *
6  * Based on asm and ptrace code by many authors.  The code here originated
7  * in ptrace.c and signal.c.
8  */
9 
10 #include <linux/kernel.h>
11 #include <linux/sched.h>
12 #include <linux/sched/task_stack.h>
13 #include <linux/entry-common.h>
14 #include <linux/mm.h>
15 #include <linux/smp.h>
16 #include <linux/errno.h>
17 #include <linux/ptrace.h>
18 #include <linux/export.h>
19 #include <linux/nospec.h>
20 #include <linux/syscalls.h>
21 #include <linux/uaccess.h>
22 
23 #ifdef CONFIG_XEN_PV
24 #include <xen/xen-ops.h>
25 #include <xen/events.h>
26 #endif
27 
28 #include <asm/desc.h>
29 #include <asm/traps.h>
30 #include <asm/vdso.h>
31 #include <asm/cpufeature.h>
32 #include <asm/fpu/api.h>
33 #include <asm/nospec-branch.h>
34 #include <asm/io_bitmap.h>
35 #include <asm/syscall.h>
36 #include <asm/irq_stack.h>
37 
38 #ifdef CONFIG_X86_64
39 
do_syscall_x64(struct pt_regs * regs,int nr)40 static __always_inline bool do_syscall_x64(struct pt_regs *regs, int nr)
41 {
42 	/*
43 	 * Convert negative numbers to very high and thus out of range
44 	 * numbers for comparisons.
45 	 */
46 	unsigned int unr = nr;
47 
48 	if (likely(unr < NR_syscalls)) {
49 		unr = array_index_nospec(unr, NR_syscalls);
50 		regs->ax = sys_call_table[unr](regs);
51 		return true;
52 	}
53 	return false;
54 }
55 
do_syscall_x32(struct pt_regs * regs,int nr)56 static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
57 {
58 	/*
59 	 * Adjust the starting offset of the table, and convert numbers
60 	 * < __X32_SYSCALL_BIT to very high and thus out of range
61 	 * numbers for comparisons.
62 	 */
63 	unsigned int xnr = nr - __X32_SYSCALL_BIT;
64 
65 	if (IS_ENABLED(CONFIG_X86_X32_ABI) && likely(xnr < X32_NR_syscalls)) {
66 		xnr = array_index_nospec(xnr, X32_NR_syscalls);
67 		regs->ax = x32_sys_call_table[xnr](regs);
68 		return true;
69 	}
70 	return false;
71 }
72 
do_syscall_64(struct pt_regs * regs,int nr)73 __visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
74 {
75 	add_random_kstack_offset();
76 	nr = syscall_enter_from_user_mode(regs, nr);
77 
78 	instrumentation_begin();
79 
80 	if (!do_syscall_x64(regs, nr) && !do_syscall_x32(regs, nr) && nr != -1) {
81 		/* Invalid system call, but still a system call. */
82 		regs->ax = __x64_sys_ni_syscall(regs);
83 	}
84 
85 	instrumentation_end();
86 	syscall_exit_to_user_mode(regs);
87 }
88 #endif
89 
90 #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
syscall_32_enter(struct pt_regs * regs)91 static __always_inline int syscall_32_enter(struct pt_regs *regs)
92 {
93 	if (IS_ENABLED(CONFIG_IA32_EMULATION))
94 		current_thread_info()->status |= TS_COMPAT;
95 
96 	return (int)regs->orig_ax;
97 }
98 
99 /*
100  * Invoke a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.
101  */
do_syscall_32_irqs_on(struct pt_regs * regs,int nr)102 static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, int nr)
103 {
104 	/*
105 	 * Convert negative numbers to very high and thus out of range
106 	 * numbers for comparisons.
107 	 */
108 	unsigned int unr = nr;
109 
110 	if (likely(unr < IA32_NR_syscalls)) {
111 		unr = array_index_nospec(unr, IA32_NR_syscalls);
112 		regs->ax = ia32_sys_call_table[unr](regs);
113 	} else if (nr != -1) {
114 		regs->ax = __ia32_sys_ni_syscall(regs);
115 	}
116 }
117 
118 /* Handles int $0x80 */
do_int80_syscall_32(struct pt_regs * regs)119 __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
120 {
121 	int nr = syscall_32_enter(regs);
122 
123 	add_random_kstack_offset();
124 	/*
125 	 * Subtlety here: if ptrace pokes something larger than 2^31-1 into
126 	 * orig_ax, the int return value truncates it. This matches
127 	 * the semantics of syscall_get_nr().
128 	 */
129 	nr = syscall_enter_from_user_mode(regs, nr);
130 	instrumentation_begin();
131 
132 	do_syscall_32_irqs_on(regs, nr);
133 
134 	instrumentation_end();
135 	syscall_exit_to_user_mode(regs);
136 }
137 
__do_fast_syscall_32(struct pt_regs * regs)138 static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
139 {
140 	int nr = syscall_32_enter(regs);
141 	int res;
142 
143 	add_random_kstack_offset();
144 	/*
145 	 * This cannot use syscall_enter_from_user_mode() as it has to
146 	 * fetch EBP before invoking any of the syscall entry work
147 	 * functions.
148 	 */
149 	syscall_enter_from_user_mode_prepare(regs);
150 
151 	instrumentation_begin();
152 	/* Fetch EBP from where the vDSO stashed it. */
153 	if (IS_ENABLED(CONFIG_X86_64)) {
154 		/*
155 		 * Micro-optimization: the pointer we're following is
156 		 * explicitly 32 bits, so it can't be out of range.
157 		 */
158 		res = __get_user(*(u32 *)&regs->bp,
159 			 (u32 __user __force *)(unsigned long)(u32)regs->sp);
160 	} else {
161 		res = get_user(*(u32 *)&regs->bp,
162 		       (u32 __user __force *)(unsigned long)(u32)regs->sp);
163 	}
164 
165 	if (res) {
166 		/* User code screwed up. */
167 		regs->ax = -EFAULT;
168 
169 		local_irq_disable();
170 		instrumentation_end();
171 		irqentry_exit_to_user_mode(regs);
172 		return false;
173 	}
174 
175 	nr = syscall_enter_from_user_mode_work(regs, nr);
176 
177 	/* Now this is just like a normal syscall. */
178 	do_syscall_32_irqs_on(regs, nr);
179 
180 	instrumentation_end();
181 	syscall_exit_to_user_mode(regs);
182 	return true;
183 }
184 
185 /* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
do_fast_syscall_32(struct pt_regs * regs)186 __visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
187 {
188 	/*
189 	 * Called using the internal vDSO SYSENTER/SYSCALL32 calling
190 	 * convention.  Adjust regs so it looks like we entered using int80.
191 	 */
192 	unsigned long landing_pad = (unsigned long)current->mm->context.vdso +
193 					vdso_image_32.sym_int80_landing_pad;
194 
195 	/*
196 	 * SYSENTER loses EIP, and even SYSCALL32 needs us to skip forward
197 	 * so that 'regs->ip -= 2' lands back on an int $0x80 instruction.
198 	 * Fix it up.
199 	 */
200 	regs->ip = landing_pad;
201 
202 	/* Invoke the syscall. If it failed, keep it simple: use IRET. */
203 	if (!__do_fast_syscall_32(regs))
204 		return 0;
205 
206 #ifdef CONFIG_X86_64
207 	/*
208 	 * Opportunistic SYSRETL: if possible, try to return using SYSRETL.
209 	 * SYSRETL is available on all 64-bit CPUs, so we don't need to
210 	 * bother with SYSEXIT.
211 	 *
212 	 * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
213 	 * because the ECX fixup above will ensure that this is essentially
214 	 * never the case.
215 	 */
216 	return regs->cs == __USER32_CS && regs->ss == __USER_DS &&
217 		regs->ip == landing_pad &&
218 		(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;
219 #else
220 	/*
221 	 * Opportunistic SYSEXIT: if possible, try to return using SYSEXIT.
222 	 *
223 	 * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
224 	 * because the ECX fixup above will ensure that this is essentially
225 	 * never the case.
226 	 *
227 	 * We don't allow syscalls at all from VM86 mode, but we still
228 	 * need to check VM, because we might be returning from sys_vm86.
229 	 */
230 	return static_cpu_has(X86_FEATURE_SEP) &&
231 		regs->cs == __USER_CS && regs->ss == __USER_DS &&
232 		regs->ip == landing_pad &&
233 		(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
234 #endif
235 }
236 
237 /* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
do_SYSENTER_32(struct pt_regs * regs)238 __visible noinstr long do_SYSENTER_32(struct pt_regs *regs)
239 {
240 	/* SYSENTER loses RSP, but the vDSO saved it in RBP. */
241 	regs->sp = regs->bp;
242 
243 	/* SYSENTER clobbers EFLAGS.IF.  Assume it was set in usermode. */
244 	regs->flags |= X86_EFLAGS_IF;
245 
246 	return do_fast_syscall_32(regs);
247 }
248 #endif
249 
SYSCALL_DEFINE0(ni_syscall)250 SYSCALL_DEFINE0(ni_syscall)
251 {
252 	return -ENOSYS;
253 }
254 
255 #ifdef CONFIG_XEN_PV
256 #ifndef CONFIG_PREEMPTION
257 /*
258  * Some hypercalls issued by the toolstack can take many 10s of
259  * seconds. Allow tasks running hypercalls via the privcmd driver to
260  * be voluntarily preempted even if full kernel preemption is
261  * disabled.
262  *
263  * Such preemptible hypercalls are bracketed by
264  * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
265  * calls.
266  */
267 DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
268 EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);
269 
270 /*
271  * In case of scheduling the flag must be cleared and restored after
272  * returning from schedule as the task might move to a different CPU.
273  */
get_and_clear_inhcall(void)274 static __always_inline bool get_and_clear_inhcall(void)
275 {
276 	bool inhcall = __this_cpu_read(xen_in_preemptible_hcall);
277 
278 	__this_cpu_write(xen_in_preemptible_hcall, false);
279 	return inhcall;
280 }
281 
restore_inhcall(bool inhcall)282 static __always_inline void restore_inhcall(bool inhcall)
283 {
284 	__this_cpu_write(xen_in_preemptible_hcall, inhcall);
285 }
286 #else
get_and_clear_inhcall(void)287 static __always_inline bool get_and_clear_inhcall(void) { return false; }
restore_inhcall(bool inhcall)288 static __always_inline void restore_inhcall(bool inhcall) { }
289 #endif
290 
__xen_pv_evtchn_do_upcall(struct pt_regs * regs)291 static void __xen_pv_evtchn_do_upcall(struct pt_regs *regs)
292 {
293 	struct pt_regs *old_regs = set_irq_regs(regs);
294 
295 	inc_irq_stat(irq_hv_callback_count);
296 
297 	xen_hvm_evtchn_do_upcall();
298 
299 	set_irq_regs(old_regs);
300 }
301 
xen_pv_evtchn_do_upcall(struct pt_regs * regs)302 __visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
303 {
304 	irqentry_state_t state = irqentry_enter(regs);
305 	bool inhcall;
306 
307 	instrumentation_begin();
308 	run_sysvec_on_irqstack_cond(__xen_pv_evtchn_do_upcall, regs);
309 
310 	inhcall = get_and_clear_inhcall();
311 	if (inhcall && !WARN_ON_ONCE(state.exit_rcu)) {
312 		irqentry_exit_cond_resched();
313 		instrumentation_end();
314 		restore_inhcall(inhcall);
315 	} else {
316 		instrumentation_end();
317 		irqentry_exit(regs, state);
318 	}
319 }
320 #endif /* CONFIG_XEN_PV */
321