@@ -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;
}
This is a note to let you know that I have just added a patch titled tty: Fix lockless tty buffer race to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree which can be found at: http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue This patch is scheduled to be released in version 3.13.11.3. If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.13.y.z tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Kamal ------ From e6227bdf60685a26bad678cfc005acf9b18000a7 Mon Sep 17 00:00:00 2001 From: Peter Hurley <peter@hurleysoftware.com> Date: Fri, 2 May 2014 10:56:12 -0400 Subject: tty: Fix lockless tty buffer race 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@gmx.at> Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Kamal Mostafa <kamal@canonical.com> --- drivers/tty/tty_buffer.c | 17 ++++++++++++++--- 1 file changed, 14 insertions(+), 3 deletions(-) -- 1.9.1