[PATCH v2 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

Eric Biggers
2020-03-18T22:40:15Z

From: Eric Biggers <ebiggers@google.com>

vt_in_use() dereferences console_driver->ttys[i] without proper locking.
This is broken because the tty can be closed and freed concurrently.

We could fix this by using 'READ_ONCE(console_driver->ttys[i]) != NULL'
and skipping the check of tty_struct::count. But, looking at
console_driver->ttys[i] isn't really appropriate anyway because even if
it is NULL the tty can still be in the process of being closed.

Instead, fix it by making vt_in_use() require console_lock() and check
whether the vt is allocated and has port refcount > 1. This works since
following the patch "vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use
virtual console" the port refcount is incremented while the vt is open.

Reproducer (very unreliable, but it worked for me after a few minutes):

#include <fcntl.h>
#include <linux/vt.h>

int main()
{
int fd, nproc;
struct vt_stat state;
char ttyname[16];

fd = open("/dev/tty10", O_RDONLY);
for (nproc = 1; nproc < 8; nproc *= 2)
fork();
for (;;) {
sprintf(ttyname, "/dev/tty%d", rand() % 8);
close(open(ttyname, O_RDONLY));
ioctl(fd, VT_GETSTATE, &state);
}
}

KASAN report:

BUG: KASAN: use-after-free in vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
BUG: KASAN: use-after-free in vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
Read of size 4 at addr ffff888065722468 by task syz-vt2/132

CPU: 0 PID: 132 Comm: syz-vt2 Not tainted 5.6.0-rc5-00130-g089b6d3654916 #13
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
Call Trace:
[...]
vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
[...]

Allocated by task 136:
[...]
kzalloc include/linux/slab.h:669 [inline]
alloc_tty_struct+0x96/0x8a0 drivers/tty/tty_io.c:2982
tty_init_dev+0x23/0x350 drivers/tty/tty_io.c:1334
tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
[...]

Freed by task 41:
[...]
kfree+0xbf/0x200 mm/slab.c:3757
free_tty_struct+0x8d/0xb0 drivers/tty/tty_io.c:177
release_one_tty+0x22d/0x2f0 drivers/tty/tty_io.c:1468
process_one_work+0x7f1/0x14b0 kernel/workqueue.c:2264
worker_thread+0x8b/0xc80 kernel/workqueue.c:2410
[...]

Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
drivers/tty/vt/vt_ioctl.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index f62f498f63c05..ca0e57b1c0b43 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -43,9 +43,11 @@ bool vt_dont_switch;

static inline bool vt_in_use(unsigned int i)
{
- extern struct tty_driver *console_driver;
+ const struct vc_data *vc = vc_cons[i].d;

- return console_driver->ttys[i] && console_driver->ttys[i]->count;
+ WARN_CONSOLE_UNLOCKED();
+
+ return vc && kref_read(&vc->port.kref) > 1;
}

static inline bool vt_busy(int i)
@@ -643,15 +645,16 @@ int vt_ioctl(struct tty_struct *tty,
struct vt_stat __user *vtstat = up;
unsigned short state, mask;

- /* Review: FIXME: Console lock ? */
if (put_user(fg_console + 1, &vtstat->v_active))
ret = -EFAULT;
else {
state = 1; /* /dev/tty0 is always open */
+ console_lock();
for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask;
++i, mask <<= 1)
if (vt_in_use(i))
state |= mask;
+ console_unlock();
ret = put_user(state, &vtstat->v_state);
}
break;
@@ -661,10 +664,11 @@ int vt_ioctl(struct tty_struct *tty,
* Returns the first available (non-opened) console.
*/
case VT_OPENQRY:
- /* FIXME: locking ? - but then this is a stupid API */
+ console_lock();
for (i = 0; i < MAX_NR_CONSOLES; ++i)
if (!vt_in_use(i))
break;
+ console_unlock();
uival = i < MAX_NR_CONSOLES ? (i+1) : -1;
goto setint;

--
2.25.1