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tty: Fix lockless tty buffer race

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Commit 6a20dbd6ca,
"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>
This commit is contained in:
Peter Hurley 2014-05-02 10:56:12 -04:00 committed by Greg Kroah-Hartman
parent 5fbf1a65dd
commit 62a0d8d7c2
1 changed files with 14 additions and 3 deletions
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17
drivers/tty/tty_buffer.c
View File

@ -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;
}