MIPS: Fix potential DOS by untrusted user app.

On a 64 bit kernel if an o32 syscall was made with a syscall number less
than 4000, we would read the function from outside of the bounds of the
syscall table.  This led to non-deterministic behavior including system
crashes.

While we were at it we reworked the 32 bit version as well to use fewer
instructions.  Both 32 and 64 bit versions are use the same code now.
Signed-off-by: Vlad Malov <Vlad.Malov@caviumnetworks.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/scall32-o32.S

@@ -262,14 +262,11 @@ bad_alignment:
 	LEAF(sys_syscall)
 	subu	t0, a0, __NR_O32_Linux	# check syscall number
 	sltiu	v0, t0, __NR_O32_Linux_syscalls + 1
+	beqz	t0, einval		# do not recurse
 	sll	t1, t0, 3
 	beqz	v0, einval
-
 	lw	t2, sys_call_table(t1)		# syscall routine
 
-	li	v1, 4000 - __NR_O32_Linux	# index of sys_syscall
-	beq	t0, v1, einval			# do not recurse
-
 	/* Some syscalls like execve get their arguments from struct pt_regs
 	   and claim zero arguments in the syscall table. Thus we have to
 	   assume the worst case and shuffle around all potential arguments.

arch/mips/kernel/scall64-o32.S

@@ -174,14 +174,12 @@ not_o32_scall:
 	END(handle_sys)
 
 LEAF(sys32_syscall)
-	sltu	v0, a0, __NR_O32_Linux + __NR_O32_Linux_syscalls + 1
+	subu	t0, a0, __NR_O32_Linux	# check syscall number
+	sltiu	v0, t0, __NR_O32_Linux_syscalls + 1
+	beqz	t0, einval		# do not recurse
+	dsll	t1, t0, 3
 	beqz	v0, einval
-
-	dsll	v0, a0, 3
-	ld	t2, (sys_call_table - (__NR_O32_Linux * 8))(v0)
-
-	li	v1, 4000		# indirect syscall number
-	beq	a0, v1, einval		# do not recurse
+	ld	t2, sys_call_table(t1)		# syscall routine
 
 	move	a0, a1			# shift argument registers
 	move	a1, a2