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#include <asm/unistd_compat_32.h>
18
19long compat_arm_syscall(struct pt_regs *regs, int scno);
20long sys_ni_syscall(void);
21
22static long do_ni_syscall(struct pt_regs *regs, int scno)
23{
24 if (is_compat_task()) {
25 long ret = compat_arm_syscall(regs, scno);
26 if (ret != -ENOSYS)
27 return ret;
28 }
29
30 return sys_ni_syscall();
31}
32
33static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
34{
35 return syscall_fn(regs);
36}
37
38static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
39 unsigned int sc_nr,
40 const syscall_fn_t syscall_table[])
41{
42 long ret;
43
44 add_random_kstack_offset();
45
46 if (scno < sc_nr) {
47 syscall_fn_t syscall_fn;
48 syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
49 ret = __invoke_syscall(regs, syscall_fn: syscall_fn);
50 } else {
51 ret = do_ni_syscall(regs, scno);
52 }
53
54 syscall_set_return_value(current, regs, error: 0, val: ret);
55
56 /*
57 * This value will get limited by KSTACK_OFFSET_MAX(), which is 10
58 * bits. The actual entropy will be further reduced by the compiler
59 * when applying stack alignment constraints: the AAPCS mandates a
60 * 16-byte aligned SP at function boundaries, which will remove the
61 * 4 low bits from any entropy chosen here.
62 *
63 * The resulting 6 bits of entropy is seen in SP[9:4].
64 */
65 choose_random_kstack_offset(get_random_u16());
66}
67
68static inline bool has_syscall_work(unsigned long flags)
69{
70 return unlikely(flags & _TIF_SYSCALL_WORK);
71}
72
73static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
74 const syscall_fn_t syscall_table[])
75{
76 unsigned long flags = read_thread_flags();
77
78 regs->orig_x0 = regs->regs[0];
79 regs->syscallno = scno;
80
81 /*
82 * BTI note:
83 * The architecture does not guarantee that SPSR.BTYPE is zero
84 * on taking an SVC, so we could return to userspace with a
85 * non-zero BTYPE after the syscall.
86 *
87 * This shouldn't matter except when userspace is explicitly
88 * doing something stupid, such as setting PROT_BTI on a page
89 * that lacks conforming BTI/PACIxSP instructions, falling
90 * through from one executable page to another with differing
91 * PROT_BTI, or messing with BTYPE via ptrace: in such cases,
92 * userspace should not be surprised if a SIGILL occurs on
93 * syscall return.
94 *
95 * So, don't touch regs->pstate & PSR_BTYPE_MASK here.
96 * (Similarly for HVC and SMC elsewhere.)
97 */
98
99 if (flags & _TIF_MTE_ASYNC_FAULT) {
100 /*
101 * Process the asynchronous tag check fault before the actual
102 * syscall. do_notify_resume() will send a signal to userspace
103 * before the syscall is restarted.
104 */
105 syscall_set_return_value(current, regs, error: -ERESTARTNOINTR, val: 0);
106 return;
107 }
108
109 if (has_syscall_work(flags)) {
110 /*
111 * The de-facto standard way to skip a system call using ptrace
112 * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
113 * suitable error code for consumption by userspace. However,
114 * this cannot be distinguished from a user-issued syscall(-1)
115 * and so we must set x0 to -ENOSYS here in case the tracer doesn't
116 * issue the skip and we fall into trace_exit with x0 preserved.
117 *
118 * This is slightly odd because it also means that if a tracer
119 * sets the system call number to -1 but does not initialise x0,
120 * then x0 will be preserved for all system calls apart from a
121 * user-issued syscall(-1). However, requesting a skip and not
122 * setting the return value is unlikely to do anything sensible
123 * anyway.
124 */
125 if (scno == NO_SYSCALL)
126 syscall_set_return_value(current, regs, error: -ENOSYS, val: 0);
127 scno = syscall_trace_enter(regs);
128 if (scno == NO_SYSCALL)
129 goto trace_exit;
130 }
131
132 invoke_syscall(regs, scno, sc_nr, syscall_table);
133
134 /*
135 * The tracing status may have changed under our feet, so we have to
136 * check again. However, if we were tracing entry, then we always trace
137 * exit regardless, as the old entry assembly did.
138 */
139 if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
140 flags = read_thread_flags();
141 if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
142 return;
143 }
144
145trace_exit:
146 syscall_trace_exit(regs);
147}
148
149void do_el0_svc(struct pt_regs *regs)
150{
151 el0_svc_common(regs, scno: regs->regs[8], __NR_syscalls, syscall_table: sys_call_table);
152}
153
154#ifdef CONFIG_COMPAT
155void do_el0_svc_compat(struct pt_regs *regs)
156{
157 el0_svc_common(regs, scno: regs->regs[7], sc_nr: __NR_compat32_syscalls,
158 syscall_table: compat_sys_call_table);
159}
160#endif
161

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