MIPS: signal: Protect against sigaltstack wraparound

If a process uses alternative signal stack by using sigaltstack(),
then that stack overflows and stack wraparound occurs.

Simple Explanation:
The accurate sp order is A,B,C,D,...
But now the sp points to A,B,C and A,B,C again.

This problem can reproduce by the following code:

  $ cat test_sigaltstack.c
  #include <stdio.h>
  #include <signal.h>
  #include <stdlib.h>
  #include <string.h>

  volatile int counter = 0;

  void print_sp()
  {
      unsigned long sp;

      __asm__ __volatile__("move %0, $sp" : "=r" (sp));
      printf("sp = 0x%08lx\n", sp);
  }

  void segv_handler()
  {
      int *c = NULL;

      print_sp();
      counter++;
      printf("%d\n", counter);

      if (counter == 23)
          abort();

      *c = 1;	// SEGV
  }

  int main()
  {
      int *c = NULL;
      char *s = malloc(SIGSTKSZ);
      stack_t stack;
      struct sigaction action;

      memset(s, 0, SIGSTKSZ);
      stack.ss_sp = s;
      stack.ss_flags = 0;
      stack.ss_size = SIGSTKSZ;
      if (sigaltstack(&stack, NULL)) {
          printf("Failed to use sigaltstack!\n");
          return -1;
      }

      memset(&action, 0, sizeof(action));
      action.sa_handler = segv_handler;
      action.sa_flags = SA_ONSTACK | SA_NODEFER;
      sigemptyset(&action.sa_mask);
      sigaction(SIGSEGV, &action, NULL);

      *c = 0;	//SEGV

      if (!s)
          free(s);

      return 0;
  }
  $ gcc test_sigaltstack.c -o test_sigaltstack
  $ ./test_sigaltstack
  sp = 0x120015c80
  1
  sp = 0x120015900
  2
  sp = 0x120015580
  3
  sp = 0x120015200
  4
  sp = 0x120014e80
  5
  sp = 0x120014b00
  6
  sp = 0x120014780
  7
  sp = 0x120014400
  8
  sp = 0x120014080
  9
  sp = 0x120013d00
  10
  sp = 0x120015c80
  11               # wraparound occurs! the 11nd output is same as 1st.
  sp = 0x120015900
  12
  sp = 0x120015580
  13
  sp = 0x120015200
  14
  sp = 0x120014e80
  15
  sp = 0x120014b00
  16
  sp = 0x120014780
  17
  sp = 0x120014400
  18
  sp = 0x120014080
  19
  sp = 0x120013d00
  20
  sp = 0x120015c80
  21                # wraparound occurs! the 21nd output is same as 1st.
  sp = 0x120015900
  22
  sp = 0x120015580
  23
  Aborted

With this patch:

  $ ./test_sigaltstack
  sp = 0x120015c80
  1
  sp = 0x120015900
  2
  sp = 0x120015580
  3
  sp = 0x120015200
  4
  sp = 0x120014e80
  5
  sp = 0x120014b00
  6
  sp = 0x120014780
  7
  sp = 0x120014400
  8
  sp = 0x120014080
  9
  Segmentation fault

If we are on the alternate signal stack and would overflow it, don't.
Return an always-bogus address instead so we will die with SIGSEGV.

This patch is similar with commit 83bd0102 ("x86: protect against
sigaltstack wraparound").
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/signal.c

@@ -562,6 +562,13 @@ void __user *get_sigframe(struct ksignal *ksig, struct pt_regs *regs,
 	/* Default to using normal stack */
 	sp = regs->regs[29];
 
+	/*
+	 * If we are on the alternate signal stack and would overflow it, don't.
+	 * Return an always-bogus address instead so we will die with SIGSEGV.
+	 */
+	if (on_sig_stack(sp) && !likely(on_sig_stack(sp - frame_size)))
+		return (void __user __force *)(-1UL);
+
 	/*
 	 * FPU emulator may have it's own trampoline active just
 	 * above the user stack, 16-bytes before the next lowest