Re: [PATCH v3 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

Jiri Slaby
2020-03-27T10:28:17Z

On 22. 03. 20, 4:43, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> 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: 4001d7b7fc27 ("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
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Acked-by: Jiri Slaby <jslaby@suse.cz>

> ---
> drivers/tty/vt/vt.c | 23 ++++++++++++++++++++++-
> drivers/tty/vt/vt_ioctl.c | 12 ++++--------
> 2 files changed, 26 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index bbc26d73209a4..309a39197be0a 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1075,6 +1075,17 @@ static void visual_deinit(struct vc_data *vc)
> module_put(vc->vc_sw->owner);
> }
>
> +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;
> @@ -1100,6 +1111,7 @@ 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);
> @@ -3250,6 +3262,7 @@ 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;
> @@ -3285,6 +3298,13 @@ 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)
> @@ -3410,7 +3430,8 @@ 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;
> diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
> index 7297997fcf04c..f62f498f63c05 100644
> --- a/drivers/tty/vt/vt_ioctl.c
> +++ b/drivers/tty/vt/vt_ioctl.c
> @@ -310,10 +310,8 @@ 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;
> }
> @@ -333,10 +331,8 @@ 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);
> }
> }
>
>

--
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