Re: [PATCH v3 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()

Jiri Slaby
2020-03-27T10:30:20Z

On 22. 03. 20, 4:43, Eric Biggers wrote:
> 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>

I cannot think of anything better with the current poor code state, so:

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

thanks,
--
--
js
suse labs