From e045fec48970df84647a47930fcf7a22ff7229c0 Mon Sep 17 00:00:00 2001 From: Jiri Olsa Date: Mon, 8 Nov 2010 19:01:47 +0100 Subject: [PATCH] tty: prevent DOS in the flush_to_ldisc There's a small window inside the flush_to_ldisc function, where the tty is unlocked and calling ldisc's receive_buf function. If in this window new buffer is added to the tty, the processing might never leave the flush_to_ldisc function. This scenario will hog the cpu, causing other tty processing starving, and making it impossible to interface the computer via tty. I was able to exploit this via pty interface by sending only control characters to the master input, causing the flush_to_ldisc to be scheduled, but never actually generate any output. To reproduce, please run multiple instances of following code. - SNIP #define _XOPEN_SOURCE #include #include #include #include #include int main(int argc, char **argv) { int i, slave, master = getpt(); char buf[8192]; sprintf(buf, "%s", ptsname(master)); grantpt(master); unlockpt(master); slave = open(buf, O_RDWR); if (slave < 0) { perror("open slave failed"); return 1; } for(i = 0; i < sizeof(buf); i++) buf[i] = rand() % 32; while(1) { write(master, buf, sizeof(buf)); } return 0; } - SNIP The attached patch (based on -next tree) fixes this by checking on the tty buffer tail. Once it's reached, the current work is rescheduled and another could run. Signed-off-by: Jiri Olsa Cc: stable Acked-by: Alan Cox Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_buffer.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c index cc1e985..d8210ca 100644 --- a/drivers/tty/tty_buffer.c +++ b/drivers/tty/tty_buffer.c @@ -413,7 +413,8 @@ static void flush_to_ldisc(struct work_struct *work) spin_lock_irqsave(&tty->buf.lock, flags); if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) { - struct tty_buffer *head; + struct tty_buffer *head, *tail = tty->buf.tail; + int seen_tail = 0; while ((head = tty->buf.head) != NULL) { int count; char *char_buf; @@ -423,6 +424,15 @@ static void flush_to_ldisc(struct work_struct *work) if (!count) { if (head->next == NULL) break; + /* + There's a possibility tty might get new buffer + added during the unlock window below. We could + end up spinning in here forever hogging the CPU + completely. To avoid this let's have a rest each + time we processed the tail buffer. + */ + if (tail == head) + seen_tail = 1; tty->buf.head = head->next; tty_buffer_free(tty, head); continue; @@ -432,7 +442,7 @@ static void flush_to_ldisc(struct work_struct *work) line discipline as we want to empty the queue */ if (test_bit(TTY_FLUSHPENDING, &tty->flags)) break; - if (!tty->receive_room) { + if (!tty->receive_room || seen_tail) { schedule_delayed_work(&tty->buf.work, 1); break; } -- 2.7.4