Re: [PATCH] pty: fix use after free of tty->driver_data

[Herton R. Krzesinski]

On Tue, Dec 15, 2015 at 04:05:09PM -0200, Herton R. Krzesinski wrote:
> On Tue, Dec 15, 2015 at 09:36:26AM -0800, Peter Hurley wrote:
> > > since in this
> > > case any of the tty->driver_data can be stale, due to all references/
> > > files being closed before (files related to ptmx/pts inodes set at
> > > tty->driver_data), we have the possibility of referencing an already
> > > freed inode.
> > 
> > As I wrote above, I believe this is the only possible circumstance
> > for which the file that is releasing could have stale pts inodes.
> > 
> > 
> > > The fix here is to keep a reference on the opened master ptmx inode.
> > > We maintain the inode referenced until the final pty_unix98_shutdown,
> > > and only pass this inode to devpts_kill_index.
> > 
> > Let me think some on your proposed solution.
> 
> Ok, let me know what you think, at least I will have to repost the patch
> with the changelog fixed, unless you think there is another/better solution
> for the issue.

Hi Peter, any news on this issue?

I gave some more thought and testing into this, and I think we simply should do
a change like below instead of my previous patch proposal:

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index a45660f..73e36bd 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -68,6 +68,7 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
 			mutex_lock(&devpts_mutex);
 			if (tty->link->driver_data)
 				devpts_pty_kill(tty->link->driver_data);
+			devpts_kill_index(tty->driver_data, tty->index);
 			mutex_unlock(&devpts_mutex);
 		}
 #endif
@@ -678,12 +679,6 @@ static void pty_unix98_remove(struct tty_driver *driver, struct tty_struct *tty)
 {
 }
 
-/* this is called once with whichever end is closed last */
-static void pty_unix98_shutdown(struct tty_struct *tty)
-{
-	devpts_kill_index(tty->driver_data, tty->index);
-}
-
 static const struct tty_operations ptm_unix98_ops = {
 	.lookup = ptm_unix98_lookup,
 	.install = pty_unix98_install,
@@ -697,7 +692,6 @@ static const struct tty_operations ptm_unix98_ops = {
 	.unthrottle = pty_unthrottle,
 	.ioctl = pty_unix98_ioctl,
 	.resize = pty_resize,
-	.shutdown = pty_unix98_shutdown,
 	.cleanup = pty_cleanup
 };
 
@@ -715,7 +709,6 @@ static const struct tty_operations pty_unix98_ops = {
 	.set_termios = pty_set_termios,
 	.start = pty_start,
 	.stop = pty_stop,
-	.shutdown = pty_unix98_shutdown,
 	.cleanup = pty_cleanup,
 };
 
-- 
2.4.3


That is, move devpts_kill_index up into pty_close(). It also resolves the
problem, while at the same time handles another problem which my previous
patch didn't catch, for example look at this other test case:

#define _XOPEN_SOURCE
#include <fcntl.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char **argv)
{
        pid_t pid;
        int ptm_fd, pty_fd, tty_fd;

        system("mkdir -p /mnt/newpts");
        system("mount -t devpts -o newinstance none /mnt/newpts");
        pid = fork();
        if (pid != 0)
                exit(0);
        daemon(1, 0);
        ptm_fd = open("/mnt/newpts/ptmx", O_RDWR);
        unlockpt(ptm_fd);
        pty_fd = open("/mnt/newpts/0", O_RDWR);
        tty_fd = open("/dev/tty", O_RDWR);
        pid = fork();
        if (pid == 0) {
                ioctl(tty_fd, TIOCNOTTY, NULL);
                setsid();
                sleep(20);
                close(pty_fd);
                close(ptm_fd);
                system("umount /mnt/newpts");
                sleep(10);
                exit(0);
        }
        sleep(10);
        return 0;
}

The idea here is to umount a pts mount while still we have /dev/tty pointing to
a pty opened...

And of course it doesn't go well with the late devpts_kill_index:

[ 1326.233991] ------------[ cut here ]------------
[ 1326.234014] WARNING: CPU: 1 PID: 2668 at lib/idr.c:1051 ida_remove+0x9b/0x130()
[ 1326.234015] ida_remove called for id=0 which is not allocated.
[ 1326.234016] 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 ip6table_filter ip6_tables binfmt_misc ppdev joydev floppy parport_pc parport serio_raw tpm_tis tpm virtio_balloon virtio_console virtio_net iosf_mbi crct10dif_pclmul crc32_pclmul pcspkr snd_hda_codec_generic i2c_piix4 snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_seq snd_seq_device snd_pcm snd_timer snd soundcore qxl ttm drm_kms_helper drm virtio_blk crc32c_intel virtio_pci virtio_ring virtio pata_acpi ata_generic [last unloaded: speedstep_lib]
[ 1326.234061] CPU: 1 PID: 2668 Comm: newpty Not tainted 4.4.0-rc7 #1
[ 1326.234062] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.1-20150318_183358- 04/01/2014
[ 1326.234065]  000000000000041b ffff8800375ffbc8 ffffffff8139aed4 0000000000000009
[ 1326.234068]  ffff8800375ffc18 000000000000041b ffff8800375ffc18 ffff8800375ffc08
[ 1326.234069]  ffffffff81096f15 ffff8800375ffbf8 ffff8801399c47e0 0000000000000000
[ 1326.234071] Call Trace:
[ 1326.234083]  [<ffffffff8139aed4>] dump_stack+0x48/0x64
[ 1326.234092]  [<ffffffff81096f15>] warn_slowpath_common+0x95/0xe0
[ 1326.234094]  [<ffffffff81097016>] warn_slowpath_fmt+0x46/0x50
[ 1326.234104]  [<ffffffff811fc292>] ? kfree+0x112/0x150
[ 1326.234105]  [<ffffffff8139c75b>] ida_remove+0x9b/0x130
[ 1326.234111]  [<ffffffff8129f3c7>] devpts_kill_index+0x57/0x80
[ 1326.234120]  [<ffffffff81480338>] pty_unix98_shutdown+0x18/0x20
[ 1326.234124]  [<ffffffff81474a2e>] release_tty+0x3e/0xe0
[ 1326.234129]  [<ffffffff8177d476>] ? mutex_lock+0x16/0x40
[ 1326.234131]  [<ffffffff81475b0a>] tty_release+0x44a/0x580
[ 1326.234135]  [<ffffffff8121e3d5>] __fput+0xb5/0x200
[ 1326.234137]  [<ffffffff8121e5ce>] ____fput+0xe/0x10
[ 1326.234143]  [<ffffffff810b2eb8>] task_work_run+0x68/0xa0
[ 1326.234145]  [<ffffffff8109a3e0>] do_exit+0x320/0x670
[ 1326.234147]  [<ffffffff810673d4>] ? __do_page_fault+0x1a4/0x450
[ 1326.234155]  [<ffffffff811361d0>] ? __audit_syscall_entry+0xb0/0x110
[ 1326.234161]  [<ffffffff81003376>] ? do_audit_syscall_entry+0x66/0x70
[ 1326.234164]  [<ffffffff8109a781>] do_group_exit+0x51/0xc0
[ 1326.234166]  [<ffffffff8109a807>] SyS_exit_group+0x17/0x20
[ 1326.234169]  [<ffffffff8177f92e>] entry_SYSCALL_64_fastpath+0x12/0x71
[ 1326.234171] ---[ end trace 23cebcbb1a28e0e8 ]---

So to avoid all this problem/headache with the late devpts_kill_index, we should
just move devpts_kill_index up into pty_close I think. I don't see any problem
with this yet.

If you are ok I would like to submit the new diff/patch above with a proper
changelog/signoff etc.

-- 
[]'s
Herton
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