CRISv32: prevent bogus restarts on sigreturn

Al Viro noted that CRIS is vulnerable to bogus restarts on sigreturn.

The fixes CRISv32 by using regs->exs as an additional indicator to
whether we should attempt to restart the syscall or not.  EXS is only
used in the sigtrap handling, and in that path we already have r9 (the
other indicator, which indicates if we're in a syscall or not) cleared.

Test case, a port of Al's ARM version from 653d48b22166db2d8 ("arm: fix
really nasty sigreturn bug"):

  #include <unistd.h>
  #include <signal.h>
  #include <stdlib.h>
  #include <sys/time.h>
  #include <errno.h>

  void f(int n)
  {
  	register int r10 asm ("r10") = n;

          __asm__ __volatile__(
  		"ba	1f	\n"
  		"nop		\n"
  		"break	8	\n"
  		"1: ba	.	\n"
  		"nop		\n"
  		:
  		: "r" (r10)
  		: "memory");
  }

  void handler1(int sig) { }
  void handler2(int sig) { raise(1); }
  void handler3(int sig) { exit(0); }

  int main(int argc, char *argv[])
  {
          struct sigaction s = {.sa_handler = handler2};
          struct itimerval t1 = { .it_value = {1} };
          struct itimerval t2 = { .it_value = {2} };

          signal(1, handler1);

          sigemptyset(&s.sa_mask);
          sigaddset(&s.sa_mask, 1);
          sigaction(SIGALRM, &s, NULL);

          signal(SIGVTALRM, handler3);

          setitimer(ITIMER_REAL, &t1, NULL);
          setitimer(ITIMER_VIRTUAL, &t2, NULL);

          f(-513); /* -ERESTARTNOINTR */

          return 0;
  }

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Link: http://lkml.kernel.org/r/20121208074429.GC4939@ZenIV.linux.org.uk
Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Jesper Nilsson <jespern@axis.com>

arch/cris/arch-v32/kernel/entry.S

@@ -147,7 +147,7 @@ system_call:
 	;; Stack-frame similar to the irq heads, which is reversed in
 	;; ret_from_sys_call.
 
-	sub.d	92, $sp		; Skip EXS and EDA.
+	sub.d	92, $sp		; Skip EDA.
 	movem	$r13, [$sp]
 	move.d	$sp, $r8
 	addq	14*4, $r8
@@ -158,8 +158,9 @@ system_call:
 	move	$ccs, $r4
 	move	$srp, $r5
 	move	$erp, $r6
+	move.d	$r9, $r7	; Store syscall number in EXS
 	subq	4, $sp
-	movem	$r6, [$r8]
+	movem	$r7, [$r8]
 	ei			; Enable interrupts while processing syscalls.
 	move.d	$r10, [$sp]
 

arch/cris/arch-v32/kernel/signal.c

@@ -72,6 +72,9 @@ restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
 	/* Make that the user-mode flag is set. */
 	regs->ccs |= (1 << (U_CCS_BITNR + CCS_SHIFT));
 
+	/* Don't perform syscall restarting */
+	regs->exs = -1;
+
 	/* Restore the old USP. */
 	err |= __get_user(old_usp, &sc->usp);
 	wrusp(old_usp);
@@ -427,6 +430,8 @@ do_signal(int canrestart, struct pt_regs *regs)
 {
 	struct ksignal ksig;
 
+	canrestart = canrestart && ((int)regs->exs >= 0);
+
 	/*
 	 * The common case should go fast, which is why this point is
 	 * reached from kernel-mode. If that's the case, just return