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    SubjectRe: [PATCH v3 1/2] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console
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    On 22. 03. 20, 4:43, Eric Biggers wrote:
    > 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>

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

    > ---
    > drivers/tty/vt/vt.c | 23 ++++++++++++++++++++++-
    > drivers/tty/vt/vt_ioctl.c | 12 ++++--------
    > 2 files changed, 26 insertions(+), 9 deletions(-)
    >
    > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
    > index bbc26d73209a4..309a39197be0a 100644
    > --- a/drivers/tty/vt/vt.c
    > +++ b/drivers/tty/vt/vt.c
    > @@ -1075,6 +1075,17 @@ static void visual_deinit(struct vc_data *vc)
    > module_put(vc->vc_sw->owner);
    > }
    >
    > +static void vc_port_destruct(struct tty_port *port)
    > +{
    > + struct vc_data *vc = container_of(port, struct vc_data, port);
    > +
    > + kfree(vc);
    > +}
    > +
    > +static const struct tty_port_operations vc_port_ops = {
    > + .destruct = vc_port_destruct,
    > +};
    > +
    > int vc_allocate(unsigned int currcons) /* return 0 on success */
    > {
    > struct vt_notifier_param param;
    > @@ -1100,6 +1111,7 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */
    >
    > vc_cons[currcons].d = vc;
    > tty_port_init(&vc->port);
    > + vc->port.ops = &vc_port_ops;
    > INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
    >
    > visual_init(vc, currcons, 1);
    > @@ -3250,6 +3262,7 @@ 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 +3298,13 @@ 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 +3430,8 @@ 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);
    > }
    > }
    >
    >


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    js
    suse labs

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