Commit 6bc9bf78 authored Mar 21, 2020 by Eric Biggers Committed by Greg Kroah-Hartman Apr 02, 2020

vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console



commit ca4463bf8438b403596edd0ec961ca0d4fbe0220 upstream.

The VT_DISALLOCATE ioctl can free a virtual console while tty_release()
is still running, causing a use-after-free in con_shutdown().  This
occurs because VT_DISALLOCATE considers a virtual console's
'struct vc_data' to be unused as soon as the corresponding tty's
refcount hits 0.  But actually it may be still being closed.

Fix this by making vc_data be reference-counted via the embedded
'struct tty_port'.  A newly allocated virtual console has refcount 1.
Opening it for the first time increments the refcount to 2.  Closing it
for the last time decrements the refcount (in tty_operations::cleanup()
so that it happens late enough), as does VT_DISALLOCATE.

Reproducer:
	#include <fcntl.h>
	#include <linux/vt.h>
	#include <sys/ioctl.h>
	#include <unistd.h>

	int main()
	{
		if (fork()) {
			for (;;)
				close(open("/dev/tty5", O_RDWR));
		} else {
			int fd = open("/dev/tty10", O_RDWR);

			for (;;)
				ioctl(fd, VT_DISALLOCATE, 5);
		}
	}

KASAN report:
	BUG: KASAN: use-after-free in con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
	Write of size 8 at addr ffff88806a4ec108 by task syz_vt/129

	CPU: 0 PID: 129 Comm: syz_vt Not tainted 5.6.0-rc2 #11
	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
	Call Trace:
	 [...]
	 con_shutdown+0x76/0x80 drivers/tty/vt/vt.c:3278
	 release_tty+0xa8/0x410 drivers/tty/tty_io.c:1514
	 tty_release_struct+0x34/0x50 drivers/tty/tty_io.c:1629
	 tty_release+0x984/0xed0 drivers/tty/tty_io.c:1789
	 [...]

	Allocated by task 129:
	 [...]
	 kzalloc include/linux/slab.h:669 [inline]
	 vc_allocate drivers/tty/vt/vt.c:1085 [inline]
	 vc_allocate+0x1ac/0x680 drivers/tty/vt/vt.c:1066
	 con_install+0x4d/0x3f0 drivers/tty/vt/vt.c:3229
	 tty_driver_install_tty drivers/tty/tty_io.c:1228 [inline]
	 tty_init_dev+0x94/0x350 drivers/tty/tty_io.c:1341
	 tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
	 tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
	 [...]

	Freed by task 130:
	 [...]
	 kfree+0xbf/0x1e0 mm/slab.c:3757
	 vt_disallocate drivers/tty/vt/vt_ioctl.c:300 [inline]
	 vt_ioctl+0x16dc/0x1e30 drivers/tty/vt/vt_ioctl.c:818
	 tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
	 [...]

Fixes: 4001d7b7 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Reported-by:  <syzbot+522643ab5729b0421998@syzkaller.appspotmail.com>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200322034305.210082-2-ebiggers@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent 35d64801

drivers/tty/vt/vt.c

+22 −1

Original line number	Diff line number	Diff line
		@@ -754,6 +754,17 @@ static void visual_init(struct vc_data *vc, int num, int init)
		vc->vc_screenbuf_size = vc->vc_rows * vc->vc_size_row;
		}

		static void vc_port_destruct(struct tty_port *port)
		{
		struct vc_data *vc = container_of(port, struct vc_data, port);

		kfree(vc);
		}

		static const struct tty_port_operations vc_port_ops = {
		.destruct = vc_port_destruct,
		};

		int vc_allocate(unsigned int currcons) /* return 0 on success */
		{
		struct vt_notifier_param param;
		@@ -779,6 +790,7 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */

		vc_cons[currcons].d = vc;
		tty_port_init(&vc->port);
		vc->port.ops = &vc_port_ops;
		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);

		visual_init(vc, currcons, 1);
		@@ -2897,6 +2909,7 @@ static int con_install(struct tty_driver driver, struct tty_struct tty)

		tty->driver_data = vc;
		vc->port.tty = tty;
		tty_port_get(&vc->port);

		if (!tty->winsize.ws_row && !tty->winsize.ws_col) {
		tty->winsize.ws_row = vc_cons[currcons].d->vc_rows;
		@@ -2932,6 +2945,13 @@ static void con_shutdown(struct tty_struct *tty)
		console_unlock();
		}

		static void con_cleanup(struct tty_struct *tty)
		{
		struct vc_data *vc = tty->driver_data;

		tty_port_put(&vc->port);
		}

		static int default_color = 7; /* white */
		static int default_italic_color = 2; // green (ASCII)
		static int default_underline_color = 3; // cyan (ASCII)
		@@ -3056,7 +3076,8 @@ static const struct tty_operations con_ops = {
		.throttle = con_throttle,
		.unthrottle = con_unthrottle,
		.resize = vt_resize,
		.shutdown = con_shutdown
		.shutdown = con_shutdown,
		.cleanup = con_cleanup,
		};

		static struct cdev vc0_cdev;

drivers/tty/vt/vt_ioctl.c

+4 −8

Original line number	Diff line number	Diff line
		@@ -313,10 +313,8 @@ static int vt_disallocate(unsigned int vc_num)
		vc = vc_deallocate(vc_num);
		console_unlock();

		if (vc && vc_num >= MIN_NR_CONSOLES) {
		tty_port_destroy(&vc->port);
		kfree(vc);
		}
		if (vc && vc_num >= MIN_NR_CONSOLES)
		tty_port_put(&vc->port);

		return ret;
		}
		@@ -336,10 +334,8 @@ static void vt_disallocate_all(void)
		console_unlock();

		for (i = 1; i < MAX_NR_CONSOLES; i++) {
		if (vc[i] && i >= MIN_NR_CONSOLES) {
		tty_port_destroy(&vc[i]->port);
		kfree(vc[i]);
		}
		if (vc[i] && i >= MIN_NR_CONSOLES)
		tty_port_put(&vc[i]->port);
		}
		}