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);
 50	} else {
 51		ret = do_ni_syscall(regs, scno);
 52	}
 53
 54	syscall_set_return_value(current, regs, 0, 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, -ERESTARTNOINTR, 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, -ENOSYS, 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, regs->regs[8], __NR_syscalls, sys_call_table);
152}
153
154#ifdef CONFIG_COMPAT
155void do_el0_svc_compat(struct pt_regs *regs)
156{
157	el0_svc_common(regs, regs->regs[7], __NR_compat32_syscalls,
158		       compat_sys_call_table);
159}
160#endif