Re: use-after-free in tty_check_change

[Peter Hurley]

Hi Dmitry,

On 11/26/2015 05:31 AM, Dmitry Vyukov wrote:
> 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
> -----------------------------------------------------------------------------

Thanks for the report. I think I have the patch that fixes this in
my out-queue as part of a larger patchset.

However, the stack backtrace below and the reported eip don't correspond
to the what I think the problem is:


> BUG: KASAN: use-after-free in tty_ioctl+0x1f06/0x2140 at addr ffff880061aa0968
                                 ^^^^^^^^^^^^^^
this offset is off the end of compilation unit for me with gcc 4.8.4

Could you generate a mixed listing file of drivers/tty/tty_io.c and attach
it please?

Regards,
Peter Hurley

> 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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