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);
49	} else {
50		ret = do_ni_syscall(regs, scno);
51	}
52
53	syscall_set_return_value(current, regs, 0, 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, -ERESTARTNOINTR, 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, -ENOSYS, 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, regs->regs[8], __NR_syscalls, sys_call_table);
153}
154
155#ifdef CONFIG_COMPAT
156void do_el0_svc_compat(struct pt_regs *regs)
157{
158	el0_svc_common(regs, regs->regs[7], __NR_compat_syscalls,
159		       compat_sys_call_table);
160}
161#endif
162