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 <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));
        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 <jolsa@redhat.com>
Cc: stable <stable@kernel.org>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 12 insertions, 2 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index cc1e9850d655..d8210ca00720 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;
                         }