From: Eric Biggers ebiggers@google.com
commit ca4463bf8438b403596edd0ec961ca0d4fbe0220 upstream.
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 Acked-by: Jiri Slaby jslaby@suse.cz Signed-off-by: Eric Biggers ebiggers@google.com Link: https://lore.kernel.org/r/20200322034305.210082-2-ebiggers@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/tty/vt/vt.c | 23 ++++++++++++++++++++++- drivers/tty/vt/vt_ioctl.c | 12 ++++-------- 2 files changed, 26 insertions(+), 9 deletions(-)
--- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -751,6 +751,17 @@ static void visual_init(struct vc_data * vc->vc_screenbuf_size = vc->vc_rows * vc->vc_size_row; }
+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; @@ -776,6 +787,7 @@ int vc_allocate(unsigned int currcons) /
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); @@ -2895,6 +2907,7 @@ static int con_install(struct tty_driver
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; @@ -2930,6 +2943,13 @@ static void con_shutdown(struct tty_stru 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) @@ -3054,7 +3074,8 @@ static const struct tty_operations con_o .throttle = con_throttle, .unthrottle = con_unthrottle, .resize = vt_resize, - .shutdown = con_shutdown + .shutdown = con_shutdown, + .cleanup = con_cleanup, };
static struct cdev vc0_cdev; --- a/drivers/tty/vt/vt_ioctl.c +++ b/drivers/tty/vt/vt_ioctl.c @@ -310,10 +310,8 @@ static int vt_disallocate(unsigned int v 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); } }