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Linux/arch/arm64/kernel/syscall.c

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  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 
 19 long compat_arm_syscall(struct pt_regs *regs, int scno);
 20 long sys_ni_syscall(void);
 21 
 22 static 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 
 33 static long __invoke_syscall(struct pt_regs *regs, syscall_fn_t syscall_fn)
 34 {
 35         return syscall_fn(regs);
 36 }
 37 
 38 static 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 
 68 static inline bool has_syscall_work(unsigned long flags)
 69 {
 70         return unlikely(flags & _TIF_SYSCALL_WORK);
 71 }
 72 
 73 static 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 
145 trace_exit:
146         syscall_trace_exit(regs);
147 }
148 
149 void 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
155 void 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
161 

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