diff options
author | Eric Biggers <ebiggers@google.com> | 2020-03-21 20:43:05 -0700 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2020-03-27 12:35:04 +0100 |
commit | 7cf64b18b0b96e751178b8d0505d8466ff5a448f (patch) | |
tree | 7dd24f363807bbd72d9cfb79bc26358d5c635fa3 /drivers/tty/vt | |
parent | ca4463bf8438b403596edd0ec961ca0d4fbe0220 (diff) |
vt: vt_ioctl: fix use-after-free in vt_in_use()
vt_in_use() dereferences console_driver->ttys[i] without proper locking.
This is broken because the tty can be closed and freed concurrently.
We could fix this by using 'READ_ONCE(console_driver->ttys[i]) != NULL'
and skipping the check of tty_struct::count. But, looking at
console_driver->ttys[i] isn't really appropriate anyway because even if
it is NULL the tty can still be in the process of being closed.
Instead, fix it by making vt_in_use() require console_lock() and check
whether the vt is allocated and has port refcount > 1. This works since
following the patch "vt: vt_ioctl: fix VT_DISALLOCATE freeing in-use
virtual console" the port refcount is incremented while the vt is open.
Reproducer (very unreliable, but it worked for me after a few minutes):
#include <fcntl.h>
#include <linux/vt.h>
int main()
{
int fd, nproc;
struct vt_stat state;
char ttyname[16];
fd = open("/dev/tty10", O_RDONLY);
for (nproc = 1; nproc < 8; nproc *= 2)
fork();
for (;;) {
sprintf(ttyname, "/dev/tty%d", rand() % 8);
close(open(ttyname, O_RDONLY));
ioctl(fd, VT_GETSTATE, &state);
}
}
KASAN report:
BUG: KASAN: use-after-free in vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
BUG: KASAN: use-after-free in vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
Read of size 4 at addr ffff888065722468 by task syz-vt2/132
CPU: 0 PID: 132 Comm: syz-vt2 Not tainted 5.6.0-rc5-00130-g089b6d3654916 #13
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
Call Trace:
[...]
vt_in_use drivers/tty/vt/vt_ioctl.c:48 [inline]
vt_ioctl+0x1ad3/0x1d70 drivers/tty/vt/vt_ioctl.c:657
tty_ioctl+0x9db/0x11b0 drivers/tty/tty_io.c:2660
[...]
Allocated by task 136:
[...]
kzalloc include/linux/slab.h:669 [inline]
alloc_tty_struct+0x96/0x8a0 drivers/tty/tty_io.c:2982
tty_init_dev+0x23/0x350 drivers/tty/tty_io.c:1334
tty_open_by_driver drivers/tty/tty_io.c:1987 [inline]
tty_open+0x3ca/0xb30 drivers/tty/tty_io.c:2035
[...]
Freed by task 41:
[...]
kfree+0xbf/0x200 mm/slab.c:3757
free_tty_struct+0x8d/0xb0 drivers/tty/tty_io.c:177
release_one_tty+0x22d/0x2f0 drivers/tty/tty_io.c:1468
process_one_work+0x7f1/0x14b0 kernel/workqueue.c:2264
worker_thread+0x8b/0xc80 kernel/workqueue.c:2410
[...]
Fixes: 4001d7b7fc27 ("vt: push down the tty lock so we can see what is left to tackle")
Cc: <stable@vger.kernel.org> # v3.4+
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20200322034305.210082-3-ebiggers@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/tty/vt')
-rw-r--r-- | drivers/tty/vt/vt_ioctl.c | 16 |
1 files changed, 12 insertions, 4 deletions
diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c index f62f498f63c0..daf61c28ba76 100644 --- a/drivers/tty/vt/vt_ioctl.c +++ b/drivers/tty/vt/vt_ioctl.c @@ -43,9 +43,15 @@ bool vt_dont_switch; static inline bool vt_in_use(unsigned int i) { - extern struct tty_driver *console_driver; + const struct vc_data *vc = vc_cons[i].d; - return console_driver->ttys[i] && console_driver->ttys[i]->count; + /* + * console_lock must be held to prevent the vc from being deallocated + * while we're checking whether it's in-use. + */ + WARN_CONSOLE_UNLOCKED(); + + return vc && kref_read(&vc->port.kref) > 1; } static inline bool vt_busy(int i) @@ -643,15 +649,16 @@ int vt_ioctl(struct tty_struct *tty, struct vt_stat __user *vtstat = up; unsigned short state, mask; - /* Review: FIXME: Console lock ? */ if (put_user(fg_console + 1, &vtstat->v_active)) ret = -EFAULT; else { state = 1; /* /dev/tty0 is always open */ + console_lock(); /* required by vt_in_use() */ for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask; ++i, mask <<= 1) if (vt_in_use(i)) state |= mask; + console_unlock(); ret = put_user(state, &vtstat->v_state); } break; @@ -661,10 +668,11 @@ int vt_ioctl(struct tty_struct *tty, * Returns the first available (non-opened) console. */ case VT_OPENQRY: - /* FIXME: locking ? - but then this is a stupid API */ + console_lock(); /* required by vt_in_use() */ for (i = 0; i < MAX_NR_CONSOLES; ++i) if (!vt_in_use(i)) break; + console_unlock(); uival = i < MAX_NR_CONSOLES ? (i+1) : -1; goto setint; |