87 lines
3.0 KiB
Diff
87 lines
3.0 KiB
Diff
Bugzilla: 1094240
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Upstream-status: 3.15 and CC'd to stable
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From 4291086b1f081b869c6d79e5b7441633dc3ace00 Mon Sep 17 00:00:00 2001
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From: Peter Hurley <peter@hurleysoftware.com>
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Date: Sat, 3 May 2014 14:04:59 +0200
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Subject: [PATCH] n_tty: Fix n_tty_write crash when echoing in raw mode
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The tty atomic_write_lock does not provide an exclusion guarantee for
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the tty driver if the termios settings are LECHO & !OPOST. And since
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it is unexpected and not allowed to call TTY buffer helpers like
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tty_insert_flip_string concurrently, this may lead to crashes when
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concurrect writers call pty_write. In that case the following two
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writers:
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* the ECHOing from a workqueue and
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* pty_write from the process
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race and can overflow the corresponding TTY buffer like follows.
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If we look into tty_insert_flip_string_fixed_flag, there is:
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int space = __tty_buffer_request_room(port, goal, flags);
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struct tty_buffer *tb = port->buf.tail;
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...
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memcpy(char_buf_ptr(tb, tb->used), chars, space);
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...
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tb->used += space;
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so the race of the two can result in something like this:
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A B
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__tty_buffer_request_room
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__tty_buffer_request_room
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memcpy(buf(tb->used), ...)
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tb->used += space;
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memcpy(buf(tb->used), ...) ->BOOM
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B's memcpy is past the tty_buffer due to the previous A's tb->used
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increment.
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Since the N_TTY line discipline input processing can output
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concurrently with a tty write, obtain the N_TTY ldisc output_lock to
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serialize echo output with normal tty writes. This ensures the tty
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buffer helper tty_insert_flip_string is not called concurrently and
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everything is fine.
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Note that this is nicely reproducible by an ordinary user using
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forkpty and some setup around that (raw termios + ECHO). And it is
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present in kernels at least after commit
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d945cb9cce20ac7143c2de8d88b187f62db99bdc (pty: Rework the pty layer to
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use the normal buffering logic) in 2.6.31-rc3.
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js: add more info to the commit log
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js: switch to bool
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js: lock unconditionally
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js: lock only the tty->ops->write call
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References: CVE-2014-0196
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Reported-and-tested-by: Jiri Slaby <jslaby@suse.cz>
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Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
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Signed-off-by: Jiri Slaby <jslaby@suse.cz>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>
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Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
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Cc: <stable@vger.kernel.org>
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Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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---
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drivers/tty/n_tty.c | 4 ++++
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1 file changed, 4 insertions(+)
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diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
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index 41fe8a047d37..fe9d129c8735 100644
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--- a/drivers/tty/n_tty.c
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+++ b/drivers/tty/n_tty.c
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@@ -2353,8 +2353,12 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
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if (tty->ops->flush_chars)
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tty->ops->flush_chars(tty);
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} else {
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+ struct n_tty_data *ldata = tty->disc_data;
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+
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while (nr > 0) {
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+ mutex_lock(&ldata->output_lock);
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c = tty->ops->write(tty, b, nr);
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+ mutex_unlock(&ldata->output_lock);
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if (c < 0) {
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retval = c;
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goto break_out;
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--
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1.9.0
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