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: 4001d7b7fc
("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>
This commit is contained in:
parent
ca4463bf84
commit
7cf64b18b0
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@ -43,9 +43,15 @@ bool vt_dont_switch;
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static inline bool vt_in_use(unsigned int i)
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static inline bool vt_in_use(unsigned int i)
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{
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{
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extern struct tty_driver *console_driver;
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const struct vc_data *vc = vc_cons[i].d;
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return console_driver->ttys[i] && console_driver->ttys[i]->count;
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/*
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* console_lock must be held to prevent the vc from being deallocated
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* while we're checking whether it's in-use.
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*/
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WARN_CONSOLE_UNLOCKED();
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return vc && kref_read(&vc->port.kref) > 1;
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}
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}
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static inline bool vt_busy(int i)
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static inline bool vt_busy(int i)
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@ -643,15 +649,16 @@ int vt_ioctl(struct tty_struct *tty,
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struct vt_stat __user *vtstat = up;
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struct vt_stat __user *vtstat = up;
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unsigned short state, mask;
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unsigned short state, mask;
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/* Review: FIXME: Console lock ? */
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if (put_user(fg_console + 1, &vtstat->v_active))
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if (put_user(fg_console + 1, &vtstat->v_active))
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ret = -EFAULT;
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ret = -EFAULT;
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else {
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else {
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state = 1; /* /dev/tty0 is always open */
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state = 1; /* /dev/tty0 is always open */
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console_lock(); /* required by vt_in_use() */
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for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask;
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for (i = 0, mask = 2; i < MAX_NR_CONSOLES && mask;
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++i, mask <<= 1)
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++i, mask <<= 1)
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if (vt_in_use(i))
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if (vt_in_use(i))
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state |= mask;
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state |= mask;
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console_unlock();
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ret = put_user(state, &vtstat->v_state);
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ret = put_user(state, &vtstat->v_state);
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}
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}
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break;
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break;
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@ -661,10 +668,11 @@ int vt_ioctl(struct tty_struct *tty,
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* Returns the first available (non-opened) console.
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* Returns the first available (non-opened) console.
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*/
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*/
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case VT_OPENQRY:
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case VT_OPENQRY:
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/* FIXME: locking ? - but then this is a stupid API */
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console_lock(); /* required by vt_in_use() */
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for (i = 0; i < MAX_NR_CONSOLES; ++i)
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for (i = 0; i < MAX_NR_CONSOLES; ++i)
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if (!vt_in_use(i))
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if (!vt_in_use(i))
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break;
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break;
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console_unlock();
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uival = i < MAX_NR_CONSOLES ? (i+1) : -1;
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uival = i < MAX_NR_CONSOLES ? (i+1) : -1;
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goto setint;
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goto setint;
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