Commit 62a0d8d7 authored May 02, 2014 by Peter Hurley Committed by Greg Kroah-Hartman May 03, 2014

tty: Fix lockless tty buffer race



Commit 6a20dbd6,
"tty: Fix race condition between __tty_buffer_request_room and flush_to_ldisc"
correctly identifies an unsafe race condition between
__tty_buffer_request_room() and flush_to_ldisc(), where the consumer
flush_to_ldisc() prematurely advances the head before consuming the
last of the data committed. For example:

           CPU 0                     |            CPU 1
__tty_buffer_request_room            | flush_to_ldisc
  ...                                |   ...
                                     |   count = head->commit - head->read
  n = tty_buffer_alloc()             |
  b->commit = b->used                |
  b->next = n                        |
                                     |   if (!count)                /* T */
                                     |     if (head->next == NULL)  /* F */
                                     |     buf->head = head->next

In this case, buf->head has been advanced but head->commit may have
been updated with a new value.

Instead of reintroducing an unnecessary lock, fix the race locklessly.
Read the commit-next pair in the reverse order of writing, which guarantees
the commit value read is the latest value written if the head is
advancing.

Reported-by: Manfred Schlaegl <manfred.schlaegl@gmx.at>
Cc: <stable@vger.kernel.org> # 3.12.x+
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 5fbf1a65

drivers/tty/tty_buffer.c

+14 −3

Original line number	Diff line number	Diff line
		@@ -258,7 +258,11 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
		n->flags = flags;
		buf->tail = n;
		b->commit = b->used;
		smp_mb();
		/* paired w/ barrier in flush_to_ldisc(); ensures the
		* latest commit value can be read before the head is
		* advanced to the next buffer
		*/
		smp_wmb();
		b->next = n;
		} else if (change)
		size = 0;
		@@ -444,17 +448,24 @@ static void flush_to_ldisc(struct work_struct *work)

		while (1) {
		struct tty_buffer *head = buf->head;
		struct tty_buffer *next;
		int count;

		/* Ldisc or user is trying to gain exclusive access */
		if (atomic_read(&buf->priority))
		break;

		next = head->next;
		/* paired w/ barrier in __tty_buffer_request_room();
		* ensures commit value read is not stale if the head
		* is advancing to the next buffer
		*/
		smp_rmb();
		count = head->commit - head->read;
		if (!count) {
		if (head->next == NULL)
		if (next == NULL)
		break;
		buf->head = head->next;
		buf->head = next;
		tty_buffer_free(port, head);
		continue;
		}