s390: restore address space when returning to user space

Unbalanced set_fs usages (e.g. early exit from a function and a
forgotten set_fs(USER_DS) call) may lead to a situation where the
secondary asce is the kernel space asce when returning to user
space. This would allow user space to modify kernel space at will.

This would only be possible with the above mentioned kernel bug,
however we can detect this and fix the secondary asce before returning
to user space.

Therefore a new TIF_ASCE_SECONDARY which is used within set_fs. When
returning to user space check if TIF_ASCE_SECONDARY is set, which
would indicate a bug. If it is set print a message to the console,
fixup the secondary asce, and then return to user space.

This is similar to what is being discussed for x86 and arm:
"[RFC] syscalls: Restore address limit after a syscall".

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 13 insertions, 10 deletions

diff --git a/arch/s390/include/asm/uaccess.h b/arch/s390/include/asm/uaccess.h
index b2988fc60f65..136932ff4250 100644
--- a/arch/s390/include/asm/uaccess.h
+++ b/arch/s390/include/asm/uaccess.h
@@ -14,6 +14,7 @@
  */
 #include <linux/sched.h>
 #include <linux/errno.h>
+#include <asm/processor.h>
 #include <asm/ctl_reg.h>
 
 #define VERIFY_READ     0
@@ -36,18 +37,20 @@
 
 #define get_ds()        (KERNEL_DS)
 #define get_fs()        (current->thread.mm_segment)
-
-#define set_fs(x)							\
-do {									\
-	unsigned long __pto;						\
-	current->thread.mm_segment = (x);				\
-	__pto = current->thread.mm_segment.ar4 ?			\
-		S390_lowcore.user_asce : S390_lowcore.kernel_asce;	\
-	__ctl_load(__pto, 7, 7);					\
-} while (0)
-
 #define segment_eq(a,b) ((a).ar4 == (b).ar4)
 
+static inline void set_fs(mm_segment_t fs)
+{
+	current->thread.mm_segment = fs;
+	if (segment_eq(fs, KERNEL_DS)) {
+		set_cpu_flag(CIF_ASCE_SECONDARY);
+		__ctl_load(S390_lowcore.kernel_asce, 7, 7);
+	} else {
+		clear_cpu_flag(CIF_ASCE_SECONDARY);
+		__ctl_load(S390_lowcore.user_asce, 7, 7);
+	}
+}
+
 static inline int __range_ok(unsigned long addr, unsigned long size)
 {
 	return 1;