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#ifdef CONFIG_COMPAT
 24	long ret;
 25	if (is_compat_task()) {
 26		ret = compat_arm_syscall(regs, scno);
 27		if (ret != -ENOSYS)
 28			return ret;
 29	}
 30#endif
 31
 32	return sys_ni_syscall();
 33}
 34
 35static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
 36{
 37	return syscall_fn(regs);
 38}
 39
 40static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 41			   unsigned int sc_nr,
 42			   const syscall_fn_t syscall_table[])
 43{
 44	long ret;
 45
 46	add_random_kstack_offset();
 47
 48	if (scno < sc_nr) {
 49		syscall_fn_t syscall_fn;
 50		syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
 51		ret = __invoke_syscall(regs, syscall_fn);
 52	} else {
 53		ret = do_ni_syscall(regs, scno);
 54	}
 55
 56	syscall_set_return_value(current, regs, 0, ret);
 57
 58	/*
 59	 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
 60	 * but not enough for arm64 stack utilization comfort. To keep
 61	 * reasonable stack head room, reduce the maximum offset to 9 bits.
 62	 *
 63	 * The actual entropy will be further reduced by the compiler when
 64	 * applying stack alignment constraints: the AAPCS mandates a
 65	 * 16-byte (i.e. 4-bit) aligned SP at function boundaries.
 66	 *
 67	 * The resulting 5 bits of entropy is seen in SP[8:4].
 68	 */
 69	choose_random_kstack_offset(get_random_u16() & 0x1FF);
 70}
 71
 72static inline bool has_syscall_work(unsigned long flags)
 73{
 74	return unlikely(flags & _TIF_SYSCALL_WORK);
 75}
 76
 77static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 78			   const syscall_fn_t syscall_table[])
 79{
 80	unsigned long flags = read_thread_flags();
 81
 82	regs->orig_x0 = regs->regs[0];
 83	regs->syscallno = scno;
 84
 85	/*
 86	 * BTI note:
 87	 * The architecture does not guarantee that SPSR.BTYPE is zero
 88	 * on taking an SVC, so we could return to userspace with a
 89	 * non-zero BTYPE after the syscall.
 90	 *
 91	 * This shouldn't matter except when userspace is explicitly
 92	 * doing something stupid, such as setting PROT_BTI on a page
 93	 * that lacks conforming BTI/PACIxSP instructions, falling
 94	 * through from one executable page to another with differing
 95	 * PROT_BTI, or messing with BTYPE via ptrace: in such cases,
 96	 * userspace should not be surprised if a SIGILL occurs on
 97	 * syscall return.
 98	 *
 99	 * So, don't touch regs->pstate & PSR_BTYPE_MASK here.
100	 * (Similarly for HVC and SMC elsewhere.)
101	 */
102
103	if (flags & _TIF_MTE_ASYNC_FAULT) {
104		/*
105		 * Process the asynchronous tag check fault before the actual
106		 * syscall. do_notify_resume() will send a signal to userspace
107		 * before the syscall is restarted.
108		 */
109		syscall_set_return_value(current, regs, -ERESTARTNOINTR, 0);
110		return;
111	}
112
113	if (has_syscall_work(flags)) {
114		/*
115		 * The de-facto standard way to skip a system call using ptrace
116		 * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
117		 * suitable error code for consumption by userspace. However,
118		 * this cannot be distinguished from a user-issued syscall(-1)
119		 * and so we must set x0 to -ENOSYS here in case the tracer doesn't
120		 * issue the skip and we fall into trace_exit with x0 preserved.
121		 *
122		 * This is slightly odd because it also means that if a tracer
123		 * sets the system call number to -1 but does not initialise x0,
124		 * then x0 will be preserved for all system calls apart from a
125		 * user-issued syscall(-1). However, requesting a skip and not
126		 * setting the return value is unlikely to do anything sensible
127		 * anyway.
128		 */
129		if (scno == NO_SYSCALL)
130			syscall_set_return_value(current, regs, -ENOSYS, 0);
131		scno = syscall_trace_enter(regs);
132		if (scno == NO_SYSCALL)
133			goto trace_exit;
134	}
135
136	invoke_syscall(regs, scno, sc_nr, syscall_table);
137
138	/*
139	 * The tracing status may have changed under our feet, so we have to
140	 * check again. However, if we were tracing entry, then we always trace
141	 * exit regardless, as the old entry assembly did.
142	 */
143	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
144		flags = read_thread_flags();
145		if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
146			return;
147	}
148
149trace_exit:
150	syscall_trace_exit(regs);
151}
152
153void do_el0_svc(struct pt_regs *regs)
154{
155	el0_svc_common(regs, regs->regs[8], __NR_syscalls, sys_call_table);
156}
157
158#ifdef CONFIG_COMPAT
159void do_el0_svc_compat(struct pt_regs *regs)
160{
161	el0_svc_common(regs, regs->regs[7], __NR_compat_syscalls,
162		       compat_sys_call_table);
163}
164#endif