| From | Andi Kleen <> | Subject | [PATCH] [93/223] tty: prevent DOS in the flush_to_ldisc | Date | Mon, 13 Dec 2010 00:46:33 +0100 (CET) |
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2.6.35-longterm review patch. If anyone has any objections, please let me know.
------------------ From: Jiri Olsa <jolsa@redhat.com>
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.
- 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> Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de> Signed-off-by: Andi Kleen <ak@linux.intel.com>
--- drivers/char/tty_buffer.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)
Index: linux/drivers/char/tty_buffer.c =================================================================== --- linux.orig/drivers/char/tty_buffer.c +++ linux/drivers/char/tty_buffer.c @@ -413,7 +413,8 @@ static void flush_to_ldisc(struct work_s 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_s 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_s 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; }
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