[34-longterm 052/209] tty: prevent DOS in the flush_to_ldisc

From: Paul Gortmaker
Date: Thu Apr 14 2011 - 13:46:30 EST

From: Jiri Olsa <jolsa@xxxxxxxxxx>

| This is a commit scheduled for the next v2.6.34 longterm release. |
| If you see a problem with using this for longterm, please comment.|

commit e045fec48970df84647a47930fcf7a22ff7229c0 upstream.

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.

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char **argv)
int i, slave, master = getpt();
char buf[8192];

sprintf(buf, "%s", ptsname(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;

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 <jolsa@xxxxxxxxxx>
Acked-by: Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
drivers/char/tty_buffer.c | 14 ++++++++++++--
1 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/char/tty_buffer.c b/drivers/char/tty_buffer.c
index 7ee5216..b370629 100644
--- a/drivers/char/tty_buffer.c
+++ b/drivers/char/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)
+ /*
+ 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);
@@ -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))
- if (!tty->receive_room) {
+ if (!tty->receive_room || seen_tail) {
schedule_delayed_work(&tty->buf.work, 1);

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