arch/arm64/kernel/syscall.c

   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