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authorPeter Hurley <peter@hurleysoftware.com>2014-05-02 10:56:12 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-05-03 18:14:28 -0400
commit62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2 (patch)
treeba32a5252d3cbfb5603da1d6391917415fb77b6b /drivers
parent5fbf1a65dd53ef313783c34a0e93a6e29def6136 (diff)
tty: Fix lockless tty buffer race
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@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>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/tty/tty_buffer.c17
1 files changed, 14 insertions, 3 deletions
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 8ebd9f88a6f6..cf78d1985cd8 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;
}