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    Subject[PATCH v3 2/2] vt: vt_ioctl: fix use-after-free in vt_in_use()
    Date
    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 | 16 ++++++++++++----
    1 file changed, 12 insertions(+), 4 deletions(-)

    diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
    index f62f498f63c05..daf61c28ba766 100644
    --- a/drivers/tty/vt/vt_ioctl.c
    +++ b/drivers/tty/vt/vt_ioctl.c
    @@ -43,9 +43,15 @@ 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;
    + /*
    + * console_lock must be held to prevent the vc from being deallocated
    + * while we're checking whether it's in-use.
    + */
    + WARN_CONSOLE_UNLOCKED();
    +
    + return vc && kref_read(&vc->port.kref) > 1;
    }

    static inline bool vt_busy(int i)
    @@ -643,15 +649,16 @@ 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(); /* required by vt_in_use() */
    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 +668,11 @@ 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(); /* required by vt_in_use() */
    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;

    --
    2.25.2
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