use-after-free in tty_check_change

[Dmitry Vyukov]

Hello,

The following program causes a use-after-free in tty_check_change:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>

void *thr(void *arg)
{
        int x = 0;
        ioctl((int)(long)arg, TIOCGETD, &x);
        return 0;
}

void tty()
{
        int fd = open("/dev/ptmx", O_RDONLY|O_SYNC);
        pthread_t th;
        pthread_create(&th, 0, thr, (void*)(long)fd);
        int x = 16;
        ioctl(fd, TIOCSETD, &x);
        pthread_join(th, 0);
}

int main()
{
        int i;

        for (i = 0; i < 10; i++) {
                if (fork() == 0) {
                        tty();
                        return 0;
                }
        }
        return 0;
}


==================================================================
BUG: KASAN: use-after-free in tty_ioctl+0x1f06/0x2140 at addr ffff880061aa0968
Read of size 8 by task a.out/6241
=============================================================================
BUG kmalloc-16 (Not tainted): kasan: bad access detected
-----------------------------------------------------------------------------

Disabling lock debugging due to kernel taint
INFO: Allocated in tty_ldisc_get.part.3+0x66/0x140 age=6 cpu=3 pid=6230
[<      none      >] kmem_cache_alloc_trace+0x1cf/0x220 mm/slub.c:2589
[<      none      >] tty_ldisc_get.part.3+0x66/0x140 drivers/tty/tty_ldisc.c:172
[<      none      >] tty_set_ldisc+0x83d/0xa70 drivers/tty/tty_ldisc.c:574
[<      none      >] tty_ioctl+0xb2a/0x2140 drivers/tty/tty_io.c:2865
[<     inline     >] spin_unlock include/linux/spinlock.h:347
[<     inline     >] ioctl_fionbio fs/ioctl.c:492
[<      none      >] do_vfs_ioctl+0x681/0xe40 fs/ioctl.c:572
[<      none      >] SyS_ioctl+0x8f/0xc0 fs/readdir.c:25
[<      none      >] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185

INFO: Freed in tty_set_ldisc+0x4c2/0xa70 age=12 cpu=3 pid=6230
[<      none      >] kfree+0x199/0x1b0 mm/slub.c:3632
[<     inline     >] tty_ldisc_restore drivers/tty/tty_ldisc.c:493
[<      none      >] tty_set_ldisc+0x4c2/0xa70 drivers/tty/tty_ldisc.c:571
[<      none      >] tty_ioctl+0xb2a/0x2140 drivers/tty/tty_io.c:2865
[<     inline     >] spin_unlock include/linux/spinlock.h:347
[<     inline     >] ioctl_fionbio fs/ioctl.c:492
[<      none      >] do_vfs_ioctl+0x681/0xe40 fs/ioctl.c:572
[<      none      >] SyS_ioctl+0x8f/0xc0 fs/readdir.c:25
[<      none      >] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185

INFO: Slab 0xffffea000186a800 objects=23 used=19 fp=0xffff880061aa12d0
flags=0x5fffc0000004080
INFO: Object 0xffff880061aa0968 @offset=2408 fp=0xffff880061aa0810
CPU: 2 PID: 6241 Comm: a.out Tainted: G    B           4.4.0-rc1+ #117
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 00000000ffffffff ffff88006db0fab8 ffffffff827450f6 ffff88003e807980
 ffff880061aa0968 ffff880061aa0000 ffff88006db0fae8 ffffffff81629404
 ffff88003e807980 ffffea000186a800 ffff880061aa0968 000000000000001b

Call Trace:
 [<ffffffff81631fde>] __asan_report_load8_noabort+0x3e/0x40
mm/kasan/report.c:280
 [<     inline     >] tty_check_change drivers/tty/tty_io.c:399
 [<ffffffff82a7b0a6>] tty_ioctl+0x1f06/0x2140 drivers/tty/tty_io.c:2831
 [<     inline     >] spin_unlock include/linux/spinlock.h:347
 [<     inline     >] ioctl_fionbio fs/ioctl.c:492
 [<ffffffff816aea91>] do_vfs_ioctl+0x681/0xe40 fs/ioctl.c:572
 [<ffffffff816af2df>] SyS_ioctl+0x8f/0xc0 fs/readdir.c:25
 [<ffffffff85415cf6>] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185
==================================================================


On commit 6ffeba9607343f15303a399bc402a538800d89d9 (Nov 24).
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