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author | Peter Hurley <peter@hurleysoftware.com> | 2014-05-02 10:56:12 -0400 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2014-05-03 18:14:28 -0400 |
commit | 62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2 (patch) | |
tree | ba32a5252d3cbfb5603da1d6391917415fb77b6b /drivers/tty | |
parent | 5fbf1a65dd53ef313783c34a0e93a6e29def6136 (diff) | |
download | op-kernel-dev-62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2.zip op-kernel-dev-62a0d8d7c2b29f92850e4ee3c38e5dfd936e92b2.tar.gz |
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/tty')
-rw-r--r-- | drivers/tty/tty_buffer.c | 17 |
1 files 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; } |