futex: Sanitize cmpxchg_futex_value_locked API

The cmpxchg_futex_value_locked API was funny in that it returned either
the original, user-exposed futex value OR an error code such as -EFAULT.
This was confusing at best, and could be a source of livelocks in places
that retry the cmpxchg_futex_value_locked after trying to fix the issue
by running fault_in_user_writeable().
    
This change makes the cmpxchg_futex_value_locked API more similar to the
get_futex_value_locked one, returning an error code and updating the
original value through a reference argument.
    
Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>  [tile]
Acked-by: Tony Luck <tony.luck@intel.com>  [ia64]
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michal Simek <monstr@monstr.eu>  [microblaze]
Acked-by: David Howells <dhowells@redhat.com> [frv]
Cc: Darren Hart <darren@dvhart.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <20110311024851.GC26122@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

5 files changed, 17 insertions, 12 deletions

diff --git a/arch/s390/include/asm/futex.h b/arch/s390/include/asm/futex.h
index 5c5d02de49e9..27ac515ef59c 100644
--- a/arch/s390/include/asm/futex.h
+++ b/arch/s390/include/asm/futex.h
@@ -39,13 +39,13 @@ static inline int futex_atomic_op_inuser (int encoded_op, int __user *uaddr)
         return ret;
 }
 
-static inline int futex_atomic_cmpxchg_inatomic(int __user *uaddr,
+static inline int futex_atomic_cmpxchg_inatomic(int *uval, int __user *uaddr,
                                                 int oldval, int newval)
 {
         if (! access_ok (VERIFY_WRITE, uaddr, sizeof(int)))
                 return -EFAULT;
 
-        return uaccess.futex_atomic_cmpxchg(uaddr, oldval, newval);
+        return uaccess.futex_atomic_cmpxchg(uval, uaddr, oldval, newval);
 }
 
 #endif /* __KERNEL__ */
diff --git a/arch/s390/include/asm/uaccess.h b/arch/s390/include/asm/uaccess.h
index d6b1ed0ec52b..549adf6a9b8b 100644
--- a/arch/s390/include/asm/uaccess.h
+++ b/arch/s390/include/asm/uaccess.h
@@ -84,7 +84,7 @@ struct uaccess_ops {
         size_t (*strnlen_user)(size_t, const char __user *);
         size_t (*strncpy_from_user)(size_t, const char __user *, char *);
         int (*futex_atomic_op)(int op, int __user *, int oparg, int *old);
-        int (*futex_atomic_cmpxchg)(int __user *, int old, int new);
+        int (*futex_atomic_cmpxchg)(int *, int __user *, int old, int new);
 };
 
 extern struct uaccess_ops uaccess;
diff --git a/arch/s390/lib/uaccess.h b/arch/s390/lib/uaccess.h
index 126011df14f1..89a80674e44b 100644
--- a/arch/s390/lib/uaccess.h
+++ b/arch/s390/lib/uaccess.h
@@ -12,12 +12,12 @@ extern size_t copy_from_user_std(size_t, const void __user *, void *);
 extern size_t copy_to_user_std(size_t, void __user *, const void *);
 extern size_t strnlen_user_std(size_t, const char __user *);
 extern size_t strncpy_from_user_std(size_t, const char __user *, char *);
-extern int futex_atomic_cmpxchg_std(int __user *, int, int);
+extern int futex_atomic_cmpxchg_std(int *, int __user *, int, int);
 extern int futex_atomic_op_std(int, int __user *, int, int *);
 
 extern size_t copy_from_user_pt(size_t, const void __user *, void *);
 extern size_t copy_to_user_pt(size_t, void __user *, const void *);
 extern int futex_atomic_op_pt(int, int __user *, int, int *);
-extern int futex_atomic_cmpxchg_pt(int __user *, int, int);
+extern int futex_atomic_cmpxchg_pt(int *, int __user *, int, int);
 
 #endif /* __ARCH_S390_LIB_UACCESS_H */
diff --git a/arch/s390/lib/uaccess_pt.c b/arch/s390/lib/uaccess_pt.c
index 404f2de296dc..b3cebcd52f5c 100644
--- a/arch/s390/lib/uaccess_pt.c
+++ b/arch/s390/lib/uaccess_pt.c
@@ -354,26 +354,29 @@ int futex_atomic_op_pt(int op, int __user *uaddr, int oparg, int *old)
         return ret;
 }
 
-static int __futex_atomic_cmpxchg_pt(int __user *uaddr, int oldval, int newval)
+static int __futex_atomic_cmpxchg_pt(int *uval, int __user *uaddr,
+                                     int oldval, int newval)
 {
         int ret;
 
         asm volatile("0: cs   %1,%4,0(%5)\n"
-                     "1: lr   %0,%1\n"
+                     "1: la   %0,0\n"
                      "2:\n"
                      EX_TABLE(0b,2b) EX_TABLE(1b,2b)
                      : "=d" (ret), "+d" (oldval), "=m" (*uaddr)
                      : "0" (-EFAULT), "d" (newval), "a" (uaddr), "m" (*uaddr)
                      : "cc", "memory" );
+        *uval = oldval;
         return ret;
 }
 
-int futex_atomic_cmpxchg_pt(int __user *uaddr, int oldval, int newval)
+int futex_atomic_cmpxchg_pt(int *uval, int __user *uaddr,
+                            int oldval, int newval)
 {
         int ret;
 
         if (segment_eq(get_fs(), KERNEL_DS))
-                return __futex_atomic_cmpxchg_pt(uaddr, oldval, newval);
+                return __futex_atomic_cmpxchg_pt(uval, uaddr, oldval, newval);
         spin_lock(&current->mm->page_table_lock);
         uaddr = (int __user *) __dat_user_addr((unsigned long) uaddr);
         if (!uaddr) {
@@ -382,7 +385,7 @@ int futex_atomic_cmpxchg_pt(int __user *uaddr, int oldval, int newval)
         }
         get_page(virt_to_page(uaddr));
         spin_unlock(&current->mm->page_table_lock);
-        ret = __futex_atomic_cmpxchg_pt(uaddr, oldval, newval);
+        ret = __futex_atomic_cmpxchg_pt(uval, uaddr, oldval, newval);
         put_page(virt_to_page(uaddr));
         return ret;
 }
diff --git a/arch/s390/lib/uaccess_std.c b/arch/s390/lib/uaccess_std.c
index a6c4f7ed24a4..1d6643c0b95f 100644
--- a/arch/s390/lib/uaccess_std.c
+++ b/arch/s390/lib/uaccess_std.c
@@ -287,19 +287,21 @@ int futex_atomic_op_std(int op, int __user *uaddr, int oparg, int *old)
         return ret;
 }
 
-int futex_atomic_cmpxchg_std(int __user *uaddr, int oldval, int newval)
+int futex_atomic_cmpxchg_std(int *uval, int __user *uaddr,
+                             int oldval, int newval)
 {
         int ret;
 
         asm volatile(
                 "   sacf 256\n"
                 "0: cs   %1,%4,0(%5)\n"
-                "1: lr   %0,%1\n"
+                "1: la   %0,0\n"
                 "2: sacf 0\n"
                 EX_TABLE(0b,2b) EX_TABLE(1b,2b)
                 : "=d" (ret), "+d" (oldval), "=m" (*uaddr)
                 : "0" (-EFAULT), "d" (newval), "a" (uaddr), "m" (*uaddr)
                 : "cc", "memory" );
+        *uval = oldval;
         return ret;
 }