[PATCH 2/2] tty: Fix lockless tty buffer race

From: Peter Hurley
Date: Fri May 02 2014 - 11:14:52 EST


Commit 6a20dbd6caa2358716136144bf524331d70b1e03,
"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@xxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.12.x+
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/tty_buffer.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 8ebd9f8..cf78d19 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -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;
}
--
1.8.1.2

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