s390/ptrace: fix PSW mask check

commit dab6cf55 upstream.

The PSW mask check of the PTRACE_POKEUSR_AREA command is incorrect.
For the default user_mode=home address space layout the psw_user_bits
variable has the home space address-space-control bits set. But the
PSW_MASK_USER contains PSW_MASK_ASC, the ptrace validity check for the
PSW mask will therefore always fail.

Fixes CVE-2014-3534
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/s390/kernel/ptrace.c

@@ -292,7 +292,9 @@ static int __poke_user(struct task_struct *child, addr_t addr, addr_t data)
 		 * psw and gprs are stored on the stack
 		 */
 		if (addr == (addr_t) &dummy->regs.psw.mask &&
-		    ((data & ~PSW_MASK_USER) != psw_user_bits ||
+		    (((data^psw_user_bits) & ~PSW_MASK_USER) ||
+		     (((data^psw_user_bits) & PSW_MASK_ASC) &&
+		      ((data|psw_user_bits) & PSW_MASK_ASC) == PSW_MASK_ASC) ||
 		     ((data & PSW_MASK_EA) && !(data & PSW_MASK_BA))))
 			/* Invalid psw mask. */
 			return -EINVAL;
@@ -595,7 +597,10 @@ static int __poke_user_compat(struct task_struct *child,
 		 */
 		if (addr == (addr_t) &dummy32->regs.psw.mask) {
 			/* Build a 64 bit psw mask from 31 bit mask. */
-			if ((tmp & ~PSW32_MASK_USER) != psw32_user_bits)
+			if (((tmp^psw32_user_bits) & ~PSW32_MASK_USER) ||
+			    (((tmp^psw32_user_bits) & PSW32_MASK_ASC) &&
+			     ((tmp|psw32_user_bits) & PSW32_MASK_ASC)
+			     == PSW32_MASK_ASC))
 				/* Invalid psw mask. */
 				return -EINVAL;
 			regs->psw.mask = (regs->psw.mask & ~PSW_MASK_USER) |