1// SPDX-License-Identifier: GPL-2.0
2
3#include <linux/compiler.h>
4#include <linux/context_tracking.h>
5#include <linux/errno.h>
6#include <linux/nospec.h>
7#include <linux/ptrace.h>
8#include <linux/randomize_kstack.h>
9#include <linux/syscalls.h>
10
11#include <asm/debug-monitors.h>
12#include <asm/exception.h>
13#include <asm/fpsimd.h>
14#include <asm/syscall.h>
15#include <asm/thread_info.h>
16#include <asm/unistd.h>
17
18long compat_arm_syscall(struct pt_regs *regs, int scno);
19long sys_ni_syscall(void);
20
21static long do_ni_syscall(struct pt_regs *regs, int scno)
22{
23 if (is_compat_task()) {
24 long ret = compat_arm_syscall(regs, scno);
25 if (ret != -ENOSYS)
26 return ret;
27 }
28
29 return sys_ni_syscall();
30}
31
32static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
33{
34 return syscall_fn(regs);
35}
36
37static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
38 unsigned int sc_nr,
39 const syscall_fn_t syscall_table[])
40{
41 long ret;
42
43 add_random_kstack_offset();
44
45 if (scno < sc_nr) {
46 syscall_fn_t syscall_fn;
47 syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
48 ret = __invoke_syscall(regs, syscall_fn: syscall_fn);
49 } else {
50 ret = do_ni_syscall(regs, scno);
51 }
52
53 syscall_set_return_value(current, regs, error: 0, val: ret);
54
55 /*
56 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
57 * but not enough for arm64 stack utilization comfort. To keep
58 * reasonable stack head room, reduce the maximum offset to 9 bits.
59 *
60 * The actual entropy will be further reduced by the compiler when
61 * applying stack alignment constraints: the AAPCS mandates a
62 * 16-byte (i.e. 4-bit) aligned SP at function boundaries.
63 *
64 * The resulting 5 bits of entropy is seen in SP[8:4].
65 */
66 choose_random_kstack_offset(get_random_u16() & 0x1FF);
67}
68
69static inline bool has_syscall_work(unsigned long flags)
70{
71 return unlikely(flags & _TIF_SYSCALL_WORK);
72}
73
74static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
75 const syscall_fn_t syscall_table[])
76{
77 unsigned long flags = read_thread_flags();
78
79 regs->orig_x0 = regs->regs[0];
80 regs->syscallno = scno;
81
82 /*
83 * BTI note:
84 * The architecture does not guarantee that SPSR.BTYPE is zero
85 * on taking an SVC, so we could return to userspace with a
86 * non-zero BTYPE after the syscall.
87 *
88 * This shouldn't matter except when userspace is explicitly
89 * doing something stupid, such as setting PROT_BTI on a page
90 * that lacks conforming BTI/PACIxSP instructions, falling
91 * through from one executable page to another with differing
92 * PROT_BTI, or messing with BTYPE via ptrace: in such cases,
93 * userspace should not be surprised if a SIGILL occurs on
94 * syscall return.
95 *
96 * So, don't touch regs->pstate & PSR_BTYPE_MASK here.
97 * (Similarly for HVC and SMC elsewhere.)
98 */
99
100 if (flags & _TIF_MTE_ASYNC_FAULT) {
101 /*
102 * Process the asynchronous tag check fault before the actual
103 * syscall. do_notify_resume() will send a signal to userspace
104 * before the syscall is restarted.
105 */
106 syscall_set_return_value(current, regs, error: -ERESTARTNOINTR, val: 0);
107 return;
108 }
109
110 if (has_syscall_work(flags)) {
111 /*
112 * The de-facto standard way to skip a system call using ptrace
113 * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
114 * suitable error code for consumption by userspace. However,
115 * this cannot be distinguished from a user-issued syscall(-1)
116 * and so we must set x0 to -ENOSYS here in case the tracer doesn't
117 * issue the skip and we fall into trace_exit with x0 preserved.
118 *
119 * This is slightly odd because it also means that if a tracer
120 * sets the system call number to -1 but does not initialise x0,
121 * then x0 will be preserved for all system calls apart from a
122 * user-issued syscall(-1). However, requesting a skip and not
123 * setting the return value is unlikely to do anything sensible
124 * anyway.
125 */
126 if (scno == NO_SYSCALL)
127 syscall_set_return_value(current, regs, error: -ENOSYS, val: 0);
128 scno = syscall_trace_enter(regs);
129 if (scno == NO_SYSCALL)
130 goto trace_exit;
131 }
132
133 invoke_syscall(regs, scno, sc_nr, syscall_table);
134
135 /*
136 * The tracing status may have changed under our feet, so we have to
137 * check again. However, if we were tracing entry, then we always trace
138 * exit regardless, as the old entry assembly did.
139 */
140 if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
141 flags = read_thread_flags();
142 if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
143 return;
144 }
145
146trace_exit:
147 syscall_trace_exit(regs);
148}
149
150void do_el0_svc(struct pt_regs *regs)
151{
152 el0_svc_common(regs, scno: regs->regs[8], __NR_syscalls, syscall_table: sys_call_table);
153}
154
155#ifdef CONFIG_COMPAT
156void do_el0_svc_compat(struct pt_regs *regs)
157{
158 el0_svc_common(regs, scno: regs->regs[7], sc_nr: __NR_compat_syscalls,
159 syscall_table: compat_sys_call_table);
160}
161#endif
162

source code of linux/arch/arm64/kernel/syscall.c