futex: setup writeable mapping for futex ops which modify user space data

The futex code installs a read only mapping via get_user_pages_fast()
even if the futex op function has to modify user space data. The
eventual fault was fixed up by futex_handle_fault() which walked the
VMA with mmap_sem held.

After the cleanup patches which removed the mmap_sem dependency of the
futex code commit 4dc5b7a36a49eff97050894cf1b3a9a02523717 (futex:
clean up fault logic) removed the private VMA walk logic from the
futex code. This change results in a stale RO mapping which is not
fixed up.

Instead of reintroducing the previous fault logic we set up the
mapping in get_user_pages_fast() read/write for all operations which
modify user space data. Also handle private futexes in the same way
and make the current unconditional access_ok(VERIFY_WRITE) depend on
the futex op.
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
CC: stable@kernel.org

kernel/futex.c

@@ -193,6 +193,7 @@ static void drop_futex_key_refs(union futex_key *key)
  * @uaddr: virtual address of the futex
  * @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
  * @key: address where result is stored.
+ * @rw: mapping needs to be read/write (values: VERIFY_READ, VERIFY_WRITE)
  *
  * Returns a negative error code or 0
  * The key words are stored in *key on success.
@@ -203,7 +204,8 @@ static void drop_futex_key_refs(union futex_key *key)
  *
  * lock_page() might sleep, the caller should not hold a spinlock.
  */
-static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
+static int
+get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
 {
 	unsigned long address = (unsigned long)uaddr;
 	struct mm_struct *mm = current->mm;
@@ -226,7 +228,7 @@ static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 	 *        but access_ok() should be faster than find_vma()
 	 */
 	if (!fshared) {
-		if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
+		if (unlikely(!access_ok(rw, uaddr, sizeof(u32))))
 			return -EFAULT;
 		key->private.mm = mm;
 		key->private.address = address;
@@ -235,7 +237,7 @@ static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 	}
 
 again:
-	err = get_user_pages_fast(address, 1, 0, &page);
+	err = get_user_pages_fast(address, 1, rw == VERIFY_WRITE, &page);
 	if (err < 0)
 		return err;
 
@@ -677,7 +679,7 @@ static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
 	if (!bitset)
 		return -EINVAL;
 
-	ret = get_futex_key(uaddr, fshared, &key);
+	ret = get_futex_key(uaddr, fshared, &key, VERIFY_READ);
 	if (unlikely(ret != 0))
 		goto out;
 
@@ -723,10 +725,10 @@ futex_wake_op(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
 	int ret, op_ret;
 
 retry:
-	ret = get_futex_key(uaddr1, fshared, &key1);
+	ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
 	if (unlikely(ret != 0))
 		goto out;
-	ret = get_futex_key(uaddr2, fshared, &key2);
+	ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
 	if (unlikely(ret != 0))
 		goto out_put_key1;
 
@@ -814,10 +816,10 @@ static int futex_requeue(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
 	int ret, drop_count = 0;
 
 retry:
-	ret = get_futex_key(uaddr1, fshared, &key1);
+	ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
 	if (unlikely(ret != 0))
 		goto out;
-	ret = get_futex_key(uaddr2, fshared, &key2);
+	ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_READ);
 	if (unlikely(ret != 0))
 		goto out_put_key1;
 
@@ -1140,7 +1142,7 @@ static int futex_wait(u32 __user *uaddr, int fshared,
 	q.bitset = bitset;
 retry:
 	q.key = FUTEX_KEY_INIT;
-	ret = get_futex_key(uaddr, fshared, &q.key);
+	ret = get_futex_key(uaddr, fshared, &q.key, VERIFY_READ);
 	if (unlikely(ret != 0))
 		goto out;
 
@@ -1330,7 +1332,7 @@ static int futex_lock_pi(u32 __user *uaddr, int fshared,
 	q.pi_state = NULL;
 retry:
 	q.key = FUTEX_KEY_INIT;
-	ret = get_futex_key(uaddr, fshared, &q.key);
+	ret = get_futex_key(uaddr, fshared, &q.key, VERIFY_WRITE);
 	if (unlikely(ret != 0))
 		goto out;
 
@@ -1594,7 +1596,7 @@ static int futex_unlock_pi(u32 __user *uaddr, int fshared)
 	if ((uval & FUTEX_TID_MASK) != task_pid_vnr(current))
 		return -EPERM;
 
-	ret = get_futex_key(uaddr, fshared, &key);
+	ret = get_futex_key(uaddr, fshared, &key, VERIFY_WRITE);
 	if (unlikely(ret != 0))
 		goto out;