[RHEL7,COMMIT] ms/vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console

Submitted by Vasily Averin on Aug. 21, 2020, 8:06 a.m.

Details

Message ID 202008210806.07L86cFi013981@vz7build.vvs.sw.ru
State New
Series "ms/vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console"
Headers show

Commit Message

Vasily Averin Aug. 21, 2020, 8:06 a.m.
The commit is pushed to "branch-rh7-3.10.0-1127.18.2.vz7.163.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1127.18.2.vz7.163.3
------>
commit b427c08947e3e42a4ef9bf304219a82361e36afa
Author: Eric Biggers <ebiggers@google.com>
Date:   Fri Aug 21 11:06:37 2020 +0300

    ms/vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console
    
    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>
    
    https://jira.sw.ru/browse/PSBM-106391
    (cherry-picked from commit ca4463bf8438b403596edd0ec961ca0d4fbe0220)
    Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
---
 drivers/tty/vt/vt.c       | 23 ++++++++++++++++++++++-
 drivers/tty/vt/vt_ioctl.c | 12 ++++--------
 2 files changed, 26 insertions(+), 9 deletions(-)

Patch hide | download patch | download mbox

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 0ee0cd5..795d786 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -767,6 +767,17 @@  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;
@@ -792,6 +803,7 @@  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);
@@ -2799,6 +2811,7 @@  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;
@@ -2834,6 +2847,13 @@  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_italic_color    = 2; // green (ASCII)
 static int default_underline_color = 3; // cyan (ASCII)
 module_param_named(italic, default_italic_color, int, S_IRUGO | S_IWUSR);
@@ -2956,7 +2976,8 @@  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 cf7ccaf..8c9d142 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -298,10 +298,8 @@  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;
 }
@@ -321,10 +319,8 @@  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);
 	}
 }