From: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
3.12-stable review patch. If anyone has any objections, please let me know.
===============
commit 62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2 upstream.
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>
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
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 2b52d807934e..4847fc57f3e2 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -248,7 +248,11 @@ int tty_buffer_request_room(struct tty_port *port, size_t size)
if ((n = tty_buffer_alloc(port, size)) != NULL) {
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
size = left;
@@ -449,17 +453,24 @@ 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;
}
--
1.9.3
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