[PATCH] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

Eric Biggers
2020-02-24T07:15:55Z

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 only considers a virtual console to be
in-use if it has a tty_struct with count > 0. But actually when
count == 0, the tty is still in the process of being closed.

Fix this by treating a virtual console as in-use if it has a tty_struct
at all, even with zero count; and by making VT_DISALLOCATE take the
tty_mutex in order to provide synchronization with release_tty().

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_ioctl.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index ee6c91ef1f6cf..57d681706fa85 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -42,7 +42,7 @@
char vt_dont_switch;
extern struct tty_driver *console_driver;

-#define VT_IS_IN_USE(i) (console_driver->ttys[i] && console_driver->ttys[i]->count)
+#define VT_IS_IN_USE(i) (console_driver->ttys[i] != NULL)
#define VT_BUSY(i) (VT_IS_IN_USE(i) || i == fg_console || vc_cons[i].d == sel_cons)

/*
@@ -288,12 +288,14 @@ static int vt_disallocate(unsigned int vc_num)
struct vc_data *vc = NULL;
int ret = 0;

+ mutex_lock(&tty_mutex); /* synchronize with release_tty() */
console_lock();
if (VT_BUSY(vc_num))
ret = -EBUSY;
else if (vc_num)
vc = vc_deallocate(vc_num);
console_unlock();
+ mutex_unlock(&tty_mutex);

if (vc && vc_num >= MIN_NR_CONSOLES) {
tty_port_destroy(&vc->port);
@@ -309,6 +311,7 @@ static void vt_disallocate_all(void)
struct vc_data *vc[MAX_NR_CONSOLES];
int i;

+ mutex_lock(&tty_mutex); /* synchronize with release_tty() */
console_lock();
for (i = 1; i < MAX_NR_CONSOLES; i++)
if (!VT_BUSY(i))
@@ -316,6 +319,7 @@ static void vt_disallocate_all(void)
else
vc[i] = NULL;
console_unlock();
+ mutex_unlock(&tty_mutex);

for (i = 1; i < MAX_NR_CONSOLES; i++) {
if (vc[i] && i >= MIN_NR_CONSOLES) {
--
2.25.1