[PATCH v2 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

Eric Biggers
2020-03-18T22:40:19Z

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>
---
drivers/tty/vt/vt.c | 14 +++++++++++++-
drivers/tty/vt/vt_ioctl.c | 12 ++++--------
2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bbc26d73209a4..ec34f1f5f3bb5 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */
tty_port_init(&vc->port);
INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);

+ /* if this wasn't the case, we'd have to implement port->ops.destruct */
+ BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
+
visual_init(vc, currcons, 1);

if (!*vc->vc_uni_pagedir_loc)
@@ -3250,6 +3253,7 @@ 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 +3289,13 @@ 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 +3421,8 @@ 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);
}
}

--
2.25.1