Re: use-after-free in tty_check_change

[Dmitry Vyukov]

On Thu, Nov 26, 2015 at 2:00 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> 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?


The source is plain 6ffeba9607343f15303a399bc402a538800d89d9, but I
use CONFIG_KASAN=y, CONFIG_KASAN_INLINE=y and gcc version 6.0.0
20151014. KASAN does affect generated code significantly.
Line numbers in the report should be correct.


> 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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