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Message-ID: <56703D34.7020106@hurleysoftware.com>
Date: Tue, 15 Dec 2015 08:17:56 -0800
From: Peter Hurley <peter@...leysoftware.com>
To: "Herton R. Krzesinski" <herton@...hat.com>,
linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>
Subject: Re: pty: fix use after free/oops at pty_unix98_shutdown
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
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