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/daifflags.h>
 12#include <asm/debug-monitors.h>
 13#include <asm/exception.h>
 14#include <asm/fpsimd.h>
 15#include <asm/syscall.h>
 16#include <asm/thread_info.h>
 17#include <asm/unistd.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#ifdef CONFIG_COMPAT
 25	long ret;
 26	if (is_compat_task()) {
 27		ret = compat_arm_syscall(regs, scno);
 28		if (ret != -ENOSYS)
 29			return ret;
 30	}
 31#endif
 32
 33	return sys_ni_syscall();
 34}
 35
 36static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
 37{
 38	return syscall_fn(regs);
 39}
 40
 41static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 42			   unsigned int sc_nr,
 43			   const syscall_fn_t syscall_table[])
 44{
 45	long ret;
 46
 47	add_random_kstack_offset();
 48
 49	if (scno < sc_nr) {
 50		syscall_fn_t syscall_fn;
 51		syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
 52		ret = __invoke_syscall(regs, syscall_fn);
 53	} else {
 54		ret = do_ni_syscall(regs, scno);
 55	}
 56
 57	syscall_set_return_value(current, regs, 0, ret);
 58
 59	/*
 60	 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
 61	 * but not enough for arm64 stack utilization comfort. To keep
 62	 * reasonable stack head room, reduce the maximum offset to 9 bits.
 63	 *
 64	 * The actual entropy will be further reduced by the compiler when
 65	 * applying stack alignment constraints: the AAPCS mandates a
 66	 * 16-byte (i.e. 4-bit) aligned SP at function boundaries.
 67	 *
 68	 * The resulting 5 bits of entropy is seen in SP[8:4].
 69	 */
 70	choose_random_kstack_offset(get_random_int() & 0x1FF);
 71}
 72
 73static inline bool has_syscall_work(unsigned long flags)
 74{
 75	return unlikely(flags & _TIF_SYSCALL_WORK);
 76}
 77
 78int syscall_trace_enter(struct pt_regs *regs);
 79void syscall_trace_exit(struct pt_regs *regs);
 80
 81static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 82			   const syscall_fn_t syscall_table[])
 83{
 84	unsigned long flags = current_thread_info()->flags;
 85
 86	regs->orig_x0 = regs->regs[0];
 87	regs->syscallno = scno;
 88
 89	/*
 90	 * BTI note:
 91	 * The architecture does not guarantee that SPSR.BTYPE is zero
 92	 * on taking an SVC, so we could return to userspace with a
 93	 * non-zero BTYPE after the syscall.
 94	 *
 95	 * This shouldn't matter except when userspace is explicitly
 96	 * doing something stupid, such as setting PROT_BTI on a page
 97	 * that lacks conforming BTI/PACIxSP instructions, falling
 98	 * through from one executable page to another with differing
 99	 * PROT_BTI, or messing with BTYPE via ptrace: in such cases,
100	 * userspace should not be surprised if a SIGILL occurs on
101	 * syscall return.
102	 *
103	 * So, don't touch regs->pstate & PSR_BTYPE_MASK here.
104	 * (Similarly for HVC and SMC elsewhere.)
105	 */
106
107	local_daif_restore(DAIF_PROCCTX);
108
109	if (flags & _TIF_MTE_ASYNC_FAULT) {
110		/*
111		 * Process the asynchronous tag check fault before the actual
112		 * syscall. do_notify_resume() will send a signal to userspace
113		 * before the syscall is restarted.
114		 */
115		syscall_set_return_value(current, regs, -ERESTARTNOINTR, 0);
116		return;
117	}
118
119	if (has_syscall_work(flags)) {
120		/*
121		 * The de-facto standard way to skip a system call using ptrace
122		 * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
123		 * suitable error code for consumption by userspace. However,
124		 * this cannot be distinguished from a user-issued syscall(-1)
125		 * and so we must set x0 to -ENOSYS here in case the tracer doesn't
126		 * issue the skip and we fall into trace_exit with x0 preserved.
127		 *
128		 * This is slightly odd because it also means that if a tracer
129		 * sets the system call number to -1 but does not initialise x0,
130		 * then x0 will be preserved for all system calls apart from a
131		 * user-issued syscall(-1). However, requesting a skip and not
132		 * setting the return value is unlikely to do anything sensible
133		 * anyway.
134		 */
135		if (scno == NO_SYSCALL)
136			syscall_set_return_value(current, regs, -ENOSYS, 0);
137		scno = syscall_trace_enter(regs);
138		if (scno == NO_SYSCALL)
139			goto trace_exit;
140	}
141
142	invoke_syscall(regs, scno, sc_nr, syscall_table);
143
144	/*
145	 * The tracing status may have changed under our feet, so we have to
146	 * check again. However, if we were tracing entry, then we always trace
147	 * exit regardless, as the old entry assembly did.
148	 */
149	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
150		local_daif_mask();
151		flags = current_thread_info()->flags;
152		if (!has_syscall_work(flags) && !(flags & _TIF_SINGLESTEP))
153			return;
154		local_daif_restore(DAIF_PROCCTX);
155	}
156
157trace_exit:
158	syscall_trace_exit(regs);
159}
160
161static inline void sve_user_discard(void)
162{
163	if (!system_supports_sve())
164		return;
165
166	clear_thread_flag(TIF_SVE);
167
168	/*
169	 * task_fpsimd_load() won't be called to update CPACR_EL1 in
170	 * ret_to_user unless TIF_FOREIGN_FPSTATE is still set, which only
171	 * happens if a context switch or kernel_neon_begin() or context
172	 * modification (sigreturn, ptrace) intervenes.
173	 * So, ensure that CPACR_EL1 is already correct for the fast-path case.
174	 */
175	sve_user_disable();
176}
177
178void do_el0_svc(struct pt_regs *regs)
179{
180	sve_user_discard();
181	el0_svc_common(regs, regs->regs[8], __NR_syscalls, sys_call_table);
182}
183
184#ifdef CONFIG_COMPAT
185void do_el0_svc_compat(struct pt_regs *regs)
186{
187	el0_svc_common(regs, regs->regs[7], __NR_compat_syscalls,
188		       compat_sys_call_table);
189}
190#endif