From: Eric Biggers <[email protected]>
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: <[email protected]> # v3.4+
Reported-by: [email protected]
Signed-off-by: Eric Biggers <[email protected]>
---
drivers/tty/vt/vt.c | 14 +++++++++++++-
drivers/tty/vt/vt_ioctl.c | 12 ++++--------
2 files changed, 17 insertions(+), 9 deletions(-)
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index bbc26d73209a4..ec34f1f5f3bb5 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */
tty_port_init(&vc->port);
INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
+ /* if this wasn't the case, we'd have to implement port->ops.destruct */
+ BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
+
visual_init(vc, currcons, 1);
if (!*vc->vc_uni_pagedir_loc)
@@ -3250,6 +3253,7 @@ 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 +3289,13 @@ 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 +3421,8 @@ 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);
}
}
--
2.25.1
On 18. 03. 20, 23:38, Eric Biggers wrote:
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */
> tty_port_init(&vc->port);
> INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
>
> + /* if this wasn't the case, we'd have to implement port->ops.destruct */
> + BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> +
This is 3 lines, implementing destruct would be like 4-5 :)? Please
implement destruct instead.
Otherwise looks good.
thanks,
--
js
suse labs
On Thu, Mar 19, 2020 at 08:36:28AM +0100, Jiri Slaby wrote:
> On 18. 03. 20, 23:38, Eric Biggers wrote:
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */
> > tty_port_init(&vc->port);
> > INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
> >
> > + /* if this wasn't the case, we'd have to implement port->ops.destruct */
> > + BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> > +
>
> This is 3 lines, implementing destruct would be like 4-5 :)? Please
> implement destruct instead.
>
> Otherwise looks good.
>
Actually implementing destruct would be 12 lines, see below. Remember there is
no tty_port_operations defined yet so we'd have to define it just for destruct.
Do you still prefer it?
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index ec34f1f5f3bb5..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;
@@ -1100,11 +1111,9 @@ 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);
- /* if this wasn't the case, we'd have to implement port->ops.destruct */
- BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
-
visual_init(vc, currcons, 1);
if (!*vc->vc_uni_pagedir_loc)
On Thu, Mar 19, 2020 at 10:10:49PM -0700, Eric Biggers wrote:
> On Thu, Mar 19, 2020 at 08:36:28AM +0100, Jiri Slaby wrote:
> > On 18. 03. 20, 23:38, Eric Biggers wrote:
> > > --- a/drivers/tty/vt/vt.c
> > > +++ b/drivers/tty/vt/vt.c
> > > @@ -1102,6 +1102,9 @@ int vc_allocate(unsigned int currcons) /* return 0 on success */
> > > tty_port_init(&vc->port);
> > > INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
> > >
> > > + /* if this wasn't the case, we'd have to implement port->ops.destruct */
> > > + BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> > > +
> >
> > This is 3 lines, implementing destruct would be like 4-5 :)? Please
> > implement destruct instead.
> >
> > Otherwise looks good.
> >
>
> Actually implementing destruct would be 12 lines, see below. Remember there is
> no tty_port_operations defined yet so we'd have to define it just for destruct.
>
> Do you still prefer it?
>
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index ec34f1f5f3bb5..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;
> @@ -1100,11 +1111,9 @@ 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);
>
> - /* if this wasn't the case, we'd have to implement port->ops.destruct */
> - BUILD_BUG_ON(offsetof(struct vc_data, port) != 0);
> -
> visual_init(vc, currcons, 1);
>
> if (!*vc->vc_uni_pagedir_loc)
Yes, this is good to have, thanks for doing this.
greg k-h