Re: pty: fix use after free/oops at pty_unix98_shutdown

[Peter Hurley <peter@hurleysoftware.com>]

Hi Herton,

On 12/14/2015 07:29 PM, Herton R. Krzesinski wrote:
> Hi,
> 
> recently I got a report of a crash at pty_unix98_shutdown. after analyzing the
> issue, I managed to create a small reproducer:
> 
> $ cat test.sh
> #!/bin/sh
> 
> while true; do
>         find /sys
>         ./dopty
>         echo 2 > /proc/sys/vm/drop_caches
>         ps aux
>         sleep 40
> done
> 
> $ cat dopty.c
> #define _XOPEN_SOURCE
> #include <unistd.h>
> #include <stdlib.h>
> #include <fcntl.h>
> #include <sys/stat.h>
> #include <sys/ioctl.h>
> #include <termios.h>
> #include <errno.h>
> 
> int main(int argc, char **argv)
> {
>         pid_t pid;
>         int ptm_fd, pty_fd, tty_fd, tout;
>         char *pty_name;
>         int c = 0;
> 
>         while (c < 100) {
>                 c++;
>                 pid = fork();
>                 if (pid != 0)
>                         continue;
>                 daemon(1, 0);
>                 ptm_fd = posix_openpt(O_RDWR);
>                 if (ptm_fd < 0)
>                         return -1;
>                 grantpt(ptm_fd);
>                 unlockpt(ptm_fd);
>                 pty_name = ptsname(ptm_fd);
>                 pty_fd = open(pty_name, O_RDWR);
>                 tty_fd = open("/dev/tty", O_RDWR);
>                 pid = fork();
>                 if (pid == 0)  {
>                         ioctl(tty_fd, TIOCNOTTY, NULL);
>                         setsid();
>                         close(pty_fd);
>                         close(ptm_fd);
>                         sleep(60);
>                         return 0;
>                 }
>                 sleep(10);
>                 return 0;
>         }
>         sleep(30);
>         return 0;
> }

Thanks for the reproducer!


> Running test.sh above, after second iteration I'm able to get the following
> oops at devpts_kill_index:
> 
> [ 1741.842926] test.sh (8786): drop_caches: 2
> [ 1812.417114] test.sh (8786): drop_caches: 2
> [ 1842.405991] BUG: unable to handle kernel NULL pointer dereference at 0000000000000060
> [ 1842.406055] IP: [<ffffffff8129f36d>] devpts_kill_index+0x1d/0x80
> [ 1842.406110] PGD 0 
> [ 1842.406123] Oops: 0000 [#1] SMP 
> [ 1842.406148] Modules linked in: 8021q mrp garp stp llc nf_conntrack_ipv4 nf_defrag_ipv4 ip6t_REJECT nf_reject_ipv6 nf_conntrack_ipv6 nf_defrag_ipv6 xt_state nf_conntrack ip6t
> able_filter ip6_tables binfmt_misc ppdev parport_pc parport floppy joydev tpm_tis tpm virtio_balloon serio_raw virtio_console virtio_net iosf_mbi crct10dif_pclmul crc32_pclmul 
> pcspkr qxl ttm drm_kms_helper drm snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_seq snd_seq_device snd_pcm snd_timer snd soundcore i2c_piix4 virt
> io_blk crc32c_intel virtio_pci virtio_ring virtio pata_acpi ata_generic [last unloaded: speedstep_lib]
> [ 1842.406383] CPU: 2 PID: 9204 Comm: dopty Not tainted 4.4.0-rc5 #1
> [ 1842.406404] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
> [ 1842.406427] task: ffff880039da3a00 ti: ffff880037a7c000 task.ti: ffff880037a7c000
> [ 1842.406445] RIP: 0010:[<ffffffff8129f36d>]  [<ffffffff8129f36d>] devpts_kill_index+0x1d/0x80
> [ 1842.406473] RSP: 0018:ffff880037a7fc98  EFLAGS: 00010282
> [ 1842.406487] RAX: 0000000000000000 RBX: ffff880039e75c00 RCX: 00000001810000fd
> [ 1842.406504] RDX: 00000000ffffffff RSI: 0000000000000004 RDI: ffff88003d0cc6a8
> [ 1842.406521] RBP: ffff880037a7fca8 R08: ffffea0000ebcc50 R09: 0000000000000001
> [ 1842.406539] R10: ffffea0000ebcc58 R11: 0000000000000000 R12: 0000000000000004
> [ 1842.406556] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> [ 1842.406573] FS:  0000000000000000(0000) GS:ffff88003fd00000(0000) knlGS:0000000000000000
> [ 1842.406611] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 1842.406634] CR2: 0000000000000060 CR3: 0000000001c0a000 CR4: 00000000001406e0
> [ 1842.406665] Stack:
> [ 1842.406677]  ffff880039e75c00 0000000000000000 ffff880037a7fcb8 ffffffff814802f8
> [ 1842.406715]  ffff880037a7fce8 ffffffff814749fe ffff880000000000 ffffffff8177ce56
> [ 1842.406785]  0000000000000000 ffff880039e75c00 ffff880037a7fd88 ffffffff81475ada
> [ 1842.406821] Call Trace:
> [ 1842.406821]  [<ffffffff814802f8>] pty_unix98_shutdown+0x18/0x20
> [ 1842.406821]  [<ffffffff814749fe>] release_tty+0x3e/0xe0
> [ 1842.406821]  [<ffffffff8177ce56>] ? mutex_lock+0x16/0x40
> [ 1842.406821]  [<ffffffff81475ada>] tty_release+0x44a/0x580
> [ 1842.406821]  [<ffffffff8121e3e5>] __fput+0xb5/0x200
> [ 1842.406821]  [<ffffffff8121e5de>] ____fput+0xe/0x10
> [ 1842.406821]  [<ffffffff810b2eb8>] task_work_run+0x68/0xa0
> [ 1842.406821]  [<ffffffff8109a3e0>] do_exit+0x320/0x670
> [ 1842.406821]  [<ffffffff810673d4>] ? __do_page_fault+0x1a4/0x450
> [ 1842.406821]  [<ffffffff811361d0>] ? __audit_syscall_entry+0xb0/0x110
> [ 1842.406821]  [<ffffffff81003376>] ? do_audit_syscall_entry+0x66/0x70
> [ 1842.406821]  [<ffffffff8109a781>] do_group_exit+0x51/0xc0
> [ 1842.406821]  [<ffffffff8109a807>] SyS_exit_group+0x17/0x20
> [ 1842.406821]  [<ffffffff8177f2ee>] entry_SYSCALL_64_fastpath+0x12/0x71
> [ 1842.406821] Code: 48 c7 c3 f4 ff ff ff eb 84 e8 10 7b df ff 55 48 89 e5 48 83 ec 10 48 89 1c 24 4c 89 64 24 08 66 66 66 66 90 48 8b 47 28 41 89 f4 <48> 81 78 60 d1 1c 00 00 74 12 48 8b 15 4a bd d6 00 31 c0 48 85 
> [ 1842.406821] RIP  [<ffffffff8129f36d>] devpts_kill_index+0x1d/0x80
> [ 1842.406821]  RSP <ffff880037a7fc98>
> [ 1842.406821] CR2: 0000000000000060
> 
> The problem above is that on final close, pty_unix98_shutdown may give an stale/
> already evicted inode from tty->driver_data. Easily to happen as the test case
> shows, in case all opened /dev/pts/N references are closed before "/dev/tty".


Yeah, I agree with your analysis here. Nothing bumps the slave inode reference for
which the current tty is a proxy.

So if the current tty is the last released, that means all the master and slave
inodes have been released. So the inode still in tty->driver_data for the slave is
stale.


> I also expect in a rare case where all ptmx references are gone/closed, this also
> could happen on final close when the master tty is given to pty_unix98_shutdown.

This logic I'm not following. If the pty master is being released, then the inode
is valid for the release() operation in-progress.

Regards,
Peter Hurley


> Following this is a proposed fix to the problem, I tested here and it fixed the
> issue.
> 
> Please take a look and review, thanks!

Looking at it now.


Regards,
Peter Hurley