tty: Fix unsafe vt paste_selection()

Convert the tty_buffer_flush() exclusion mechanism to a
public interface - tty_buffer_lock/unlock_exclusive() - and use
the interface to safely write the paste selection to the line
discipline.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 56 insertions, 16 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index dbe4a718e2dc..f22e116db105 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -30,6 +30,42 @@
 
 
 /**
+ *      tty_buffer_lock_exclusive       -       gain exclusive access to buffer
+ *      tty_buffer_unlock_exclusive     -       release exclusive access
+ *
+ *      @port - tty_port owning the flip buffer
+ *
+ *      Guarantees safe use of the line discipline's receive_buf() method by
+ *      excluding the buffer work and any pending flush from using the flip
+ *      buffer. Data can continue to be added concurrently to the flip buffer
+ *      from the driver side.
+ *
+ *      On release, the buffer work is restarted if there is data in the
+ *      flip buffer
+ */
+
+void tty_buffer_lock_exclusive(struct tty_port *port)
+{
+        struct tty_bufhead *buf = &port->buf;
+
+        atomic_inc(&buf->priority);
+        mutex_lock(&buf->lock);
+}
+
+void tty_buffer_unlock_exclusive(struct tty_port *port)
+{
+        struct tty_bufhead *buf = &port->buf;
+        int restart;
+
+        restart = buf->head->commit != buf->head->read;
+
+        atomic_dec(&buf->priority);
+        mutex_unlock(&buf->lock);
+        if (restart)
+                queue_work(system_unbound_wq, &buf->work);
+}
+
+/**
  *      tty_buffer_space_avail  -       return unused buffer space
  *      @port - tty_port owning the flip buffer
  *
@@ -158,7 +194,7 @@ static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
  *      being processed by flush_to_ldisc then we defer the processing
  *      to that function
  *
- *      Locking: takes flush_mutex to ensure single-threaded flip buffer
+ *      Locking: takes buffer lock to ensure single-threaded flip buffer
  *               'consumer'
  */
 
@@ -168,16 +204,16 @@ void tty_buffer_flush(struct tty_struct *tty)
         struct tty_bufhead *buf = &port->buf;
         struct tty_buffer *next;
 
-        buf->flushpending = 1;
+        atomic_inc(&buf->priority);
 
-        mutex_lock(&buf->flush_mutex);
+        mutex_lock(&buf->lock);
         while ((next = buf->head->next) != NULL) {
                 tty_buffer_free(port, buf->head);
                 buf->head = next;
         }
         buf->head->read = buf->head->commit;
-        buf->flushpending = 0;
-        mutex_unlock(&buf->flush_mutex);
+        atomic_dec(&buf->priority);
+        mutex_unlock(&buf->lock);
 }
 
 /**
@@ -383,7 +419,7 @@ receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
  *
  *      The receive_buf method is single threaded for each tty instance.
  *
- *      Locking: takes flush_mutex to ensure single-threaded flip buffer
+ *      Locking: takes buffer lock to ensure single-threaded flip buffer
  *               'consumer'
  */
 
@@ -402,14 +438,14 @@ static void flush_to_ldisc(struct work_struct *work)
         if (disc == NULL)
                 return;
 
-        mutex_lock(&buf->flush_mutex);
+        mutex_lock(&buf->lock);
 
         while (1) {
                 struct tty_buffer *head = buf->head;
                 int count;
 
-                /* Ldisc or user is trying to flush the buffers. */
-                if (buf->flushpending)
+                /* Ldisc or user is trying to gain exclusive access */
+                if (atomic_read(&buf->priority))
                         break;
 
                 count = head->commit - head->read;
@@ -426,7 +462,7 @@ static void flush_to_ldisc(struct work_struct *work)
                         break;
         }
 
-        mutex_unlock(&buf->flush_mutex);
+        mutex_unlock(&buf->lock);
 
         tty_ldisc_deref(disc);
 }
@@ -482,13 +518,12 @@ void tty_buffer_init(struct tty_port *port)
 {
         struct tty_bufhead *buf = &port->buf;
 
-        mutex_init(&buf->flush_mutex);
+        mutex_init(&buf->lock);
         tty_buffer_reset(&buf->sentinel, 0);
         buf->head = &buf->sentinel;
         buf->tail = &buf->sentinel;
         init_llist_head(&buf->free);
         atomic_set(&buf->memory_used, 0);
-        buf->flushpending = 0;
+        atomic_set(&buf->priority, 0);
         INIT_WORK(&buf->work, flush_to_ldisc);
 }
-
diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 2ca8d6b6514c..ea27804d87af 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -24,6 +24,7 @@
 #include <linux/selection.h>
 #include <linux/tiocl.h>
 #include <linux/console.h>
+#include <linux/tty_flip.h>
 
 /* Don't take this from <ctype.h>: 011-015 on the screen aren't spaces */
 #define isspace(c)      ((c) == ' ')
@@ -346,8 +347,8 @@ int paste_selection(struct tty_struct *tty)
         console_unlock();
 
         ld = tty_ldisc_ref_wait(tty);
+        tty_buffer_lock_exclusive(&vc->port);
 
-        /* FIXME: this is completely unsafe */
         add_wait_queue(&vc->paste_wait, &wait);
         while (sel_buffer && sel_buffer_lth > pasted) {
                 set_current_state(TASK_INTERRUPTIBLE);
@@ -363,6 +364,7 @@ int paste_selection(struct tty_struct *tty)
         remove_wait_queue(&vc->paste_wait, &wait);
         __set_current_state(TASK_RUNNING);
 
+        tty_buffer_unlock_exclusive(&vc->port);
         tty_ldisc_deref(ld);
         return 0;
 }
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 991575fe3451..7a9a3b0a6b5a 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -67,8 +67,8 @@ static inline char *flag_buf_ptr(struct tty_buffer *b, int ofs)
 struct tty_bufhead {
         struct tty_buffer *head;        /* Queue head */
         struct work_struct work;
-        struct mutex flush_mutex;
-        unsigned int flushpending:1;
+        struct mutex       lock;
+        atomic_t           priority;
         struct tty_buffer sentinel;
         struct llist_head free;         /* Free queue head */
         atomic_t           memory_used; /* In-use buffers excluding free list */
diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h
index 6944ed2ce692..21ddd7d9ea1f 100644
--- a/include/linux/tty_flip.h
+++ b/include/linux/tty_flip.h
@@ -32,4 +32,7 @@ static inline int tty_insert_flip_string(struct tty_port *port,
         return tty_insert_flip_string_fixed_flag(port, chars, TTY_NORMAL, size);
 }
 
+extern void tty_buffer_lock_exclusive(struct tty_port *port);
+extern void tty_buffer_unlock_exclusive(struct tty_port *port);
+
 #endif /* _LINUX_TTY_FLIP_H */