Messages in this thread Patch in this message | | | From | Eric Biggers <> | Subject | [PATCH] vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use virtual console | Date | Sun, 23 Feb 2020 23:12:47 -0800 |
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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 only considers a virtual console to be in-use if it has a tty_struct with count > 0. But actually when count == 0, the tty is still in the process of being closed.
Fix this by treating a virtual console as in-use if it has a tty_struct at all, even with zero count; and by making VT_DISALLOCATE take the tty_mutex in order to provide synchronization with release_tty().
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> --- drivers/tty/vt/vt_ioctl.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c index ee6c91ef1f6cf..57d681706fa85 100644 --- a/drivers/tty/vt/vt_ioctl.c +++ b/drivers/tty/vt/vt_ioctl.c @@ -42,7 +42,7 @@ char vt_dont_switch; extern struct tty_driver *console_driver; -#define VT_IS_IN_USE(i) (console_driver->ttys[i] && console_driver->ttys[i]->count) +#define VT_IS_IN_USE(i) (console_driver->ttys[i] != NULL) #define VT_BUSY(i) (VT_IS_IN_USE(i) || i == fg_console || vc_cons[i].d == sel_cons) /* @@ -288,12 +288,14 @@ static int vt_disallocate(unsigned int vc_num) struct vc_data *vc = NULL; int ret = 0; + mutex_lock(&tty_mutex); /* synchronize with release_tty() */ console_lock(); if (VT_BUSY(vc_num)) ret = -EBUSY; else if (vc_num) vc = vc_deallocate(vc_num); console_unlock(); + mutex_unlock(&tty_mutex); if (vc && vc_num >= MIN_NR_CONSOLES) { tty_port_destroy(&vc->port); @@ -309,6 +311,7 @@ static void vt_disallocate_all(void) struct vc_data *vc[MAX_NR_CONSOLES]; int i; + mutex_lock(&tty_mutex); /* synchronize with release_tty() */ console_lock(); for (i = 1; i < MAX_NR_CONSOLES; i++) if (!VT_BUSY(i)) @@ -316,6 +319,7 @@ static void vt_disallocate_all(void) else vc[i] = NULL; console_unlock(); + mutex_unlock(&tty_mutex); for (i = 1; i < MAX_NR_CONSOLES; i++) { if (vc[i] && i >= MIN_NR_CONSOLES) { -- 2.25.1
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