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Date: Thu, 26 Nov 2015 08:41:47 -0500
From: Peter Hurley <peter@...leysoftware.com>
To: Dmitry Vyukov <dvyukov@...gle.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>,
LKML <linux-kernel@...r.kernel.org>
Cc: Kostya Serebryany <kcc@...gle.com>,
Alexander Potapenko <glider@...gle.com>,
Eric Dumazet <edumazet@...gle.com>,
Sasha Levin <sasha.levin@...cle.com>
Subject: Re: use-after-free in n_tty_read
Hi Dmitry,
On 11/26/2015 08:34 AM, Dmitry Vyukov wrote:
> Hello,
>
> I've hit the following report once after booting a VM and then scp a
> file into it. It is not reproducible. But maybe the stacks will give
> you some hint as to how it could happen:
>
>
> ==================================================================
> BUG: KASAN: use-after-free in n_tty_read+0x1d53/0x1e60 at addr ffff880061ec3ae0
> Read of size 8 by task sshd/5796
> =============================================================================
> BUG kmalloc-16 (Not tainted): kasan: bad access detected
> -----------------------------------------------------------------------------
Thanks for the report. I have this fix in my out-queue, attached below
for reference (which I'll now mark for -stable).
Regards,
Peter Hurley
> Disabling lock debugging due to kernel taint
> INFO: Allocated in tty_ldisc_get.part.3+0x61/0x130 age=11 cpu=3 pid=5796
> [< none >] ___slab_alloc+0x489/0x4e0 mm/slub.c:2438
> [< none >] __slab_alloc+0x4c/0x90 mm/slub.c:2467
> [< inline >] slab_alloc_node mm/slub.c:2530
> [< inline >] slab_alloc mm/slub.c:2572
> [< none >] kmem_cache_alloc_trace+0x1c6/0x210 mm/slub.c:2589
> [< inline >] kmalloc include/linux/slab.h:458
> [< none >] tty_ldisc_get.part.3+0x61/0x130 drivers/tty/tty_ldisc.c:171
> [< inline >] tty_ldisc_get drivers/tty/tty_ldisc.c:801
> [< none >] tty_ldisc_init+0x13/0x70 drivers/tty/tty_ldisc.c:802
> [< none >] alloc_tty_struct+0x103/0x810 drivers/tty/tty_io.c:3115
> [< none >] pty_common_install+0x1a2/0x940 drivers/tty/pty.c:399
> [< none >] pty_unix98_install+0xb/0x10 drivers/tty/pty.c:674
> [< inline >] tty_driver_install_tty drivers/tty/tty_io.c:1430
> [< none >] tty_init_dev+0xdb/0x3e0 drivers/tty/tty_io.c:1529
> [< none >] ptmx_open+0xbc/0x2c0 drivers/tty/pty.c:762
> [< none >] chrdev_open+0x1ef/0x570 fs/char_dev.c:388
> [< none >] do_dentry_open+0x5d2/0xab0 fs/open.c:736
> [< none >] vfs_open+0x166/0x1e0 fs/open.c:853
> [< inline >] do_last fs/namei.c:3192
> [< none >] path_openat+0xa5c/0x5e00 fs/namei.c:3324
> [< none >] do_filp_open+0x170/0x230 fs/namei.c:3359
> [< none >] do_sys_open+0x180/0x360 fs/open.c:1025
>
> INFO: Freed in tty_ldisc_reinit+0xc8/0x1b0 age=7 cpu=0 pid=5798
> [< none >] __slab_free+0x206/0x360 mm/slub.c:2648 (discriminator 1)
> [< inline >] slab_free mm/slub.c:2803
> [< none >] kfree+0x1a2/0x1c0 mm/slub.c:3632
> [< inline >] tty_ldisc_put drivers/tty/tty_ldisc.c:194
> [< none >] tty_ldisc_reinit+0xc8/0x1b0 drivers/tty/tty_ldisc.c:635
> [< none >] tty_ldisc_hangup+0x1fa/0x5f0 drivers/tty/tty_ldisc.c:708
> [< none >] __tty_hangup+0x423/0xad0 drivers/tty/tty_io.c:729
> [< inline >] tty_vhangup drivers/tty/tty_io.c:802
> [< none >] tty_vhangup_self+0x1c/0x40 drivers/tty/tty_io.c:820
> [< none >] sys_vhangup+0x1e/0x30 fs/open.c:1121
> [< none >] entry_SYSCALL_64_fastpath+0x16/0x7a
> arch/x86/entry/entry_64.S:185
> INFO: Slab 0xffffea000187b080 objects=23 used=16 fp=0xffff880061ec3ae0
> flags=0x5fffc0000004080
> INFO: Object 0xffff880061ec3ae0 @offset=6880 fp=0xffff880061ec3580
>
> Bytes b4 ffff880061ec3ad0: f1 c1 fb ff 00 00 00 00 5a 5a 5a 5a 5a 5a
> 5a 5a ........ZZZZZZZZ
> Object ffff880061ec3ae0: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b
> a5 kkkkkkkkkkkkkkk.
> Redzone ffff880061ec3af0: bb bb bb bb bb bb bb bb
> ........
> Padding ffff880061ec3c30: 5a 5a 5a 5a 5a 5a 5a 5a
> ZZZZZZZZ
> CPU: 3 PID: 5796 Comm: sshd Tainted: G B 4.4.0-rc2+ #51
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> ffff880061ec2000 ffff88006a377928 ffffffff826c8be0 ffff88003e807980
> ffff88006a377958 ffffffff815f1604 ffff88003e807980 ffffea000187b080
> ffff880061ec3ae0 ffff8800620c4670 ffff88006a377980 ffffffff815f775f
> Call Trace:
> [< inline >] __dump_stack lib/dump_stack.c:15
> [<ffffffff826c8be0>] dump_stack+0x44/0x64 lib/dump_stack.c:50
> [<ffffffff815f1604>] print_trailer+0xf4/0x150 mm/slub.c:652
> [<ffffffff815f775f>] object_err+0x2f/0x40 mm/slub.c:659
> [< inline >] print_address_description mm/kasan/report.c:138
> [<ffffffff815f9ee0>] kasan_report_error+0x210/0x520 mm/kasan/report.c:236
> [< inline >] kasan_report mm/kasan/report.c:259
> [<ffffffff815fa2ee>] __asan_report_load8_noabort+0x3e/0x40
> mm/kasan/report.c:280
> [< inline >] n_tty_check_unthrottle drivers/tty/n_tty.c:262
> [<ffffffff829e73b3>] n_tty_read+0x1d53/0x1e60 drivers/tty/n_tty.c:2302
> [<ffffffff829d8596>] tty_read+0x146/0x230 drivers/tty/tty_io.c:1071
> [<ffffffff81633bcb>] __vfs_read+0xdb/0x490 fs/read_write.c:432
> [<ffffffff81635edb>] vfs_read+0xdb/0x2d0 fs/read_write.c:454
> [< inline >] SYSC_read fs/read_write.c:569
> [<ffffffff81638c1c>] SyS_read+0x10c/0x220 fs/read_write.c:562
> [<ffffffff852a7876>] entry_SYSCALL_64_fastpath+0x16/0x7a
> arch/x86/entry/entry_64.S:185
> ==================================================================
>
>
> On commit 6ffeba9607343f15303a399bc402a538800d89d9.
>
--- >% ---
Subject: [PATCH] n_tty: Fix unsafe reference to "other" ldisc
Although n_tty_check_unthrottle() has a valid ldisc reference (since
the tty core gets the ldisc ref in tty_read() before calling the line
discipline read() method), it does not have a valid ldisc reference to
the "other" pty of a pty pair. Since getting an ldisc reference for
tty->link essentially open-codes tty_wakeup(), just replace with the
equivalent tty_wakeup().
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
drivers/tty/n_tty.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 1dcd10b..58e009d 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -261,16 +261,13 @@ static void n_tty_check_throttle(struct tty_struct *tty)
static void n_tty_check_unthrottle(struct tty_struct *tty)
{
- if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
- tty->link->ldisc->ops->write_wakeup == n_tty_write_wakeup) {
+ if (tty->driver->type == TTY_DRIVER_TYPE_PTY) {
if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
return;
if (!tty->count)
return;
n_tty_kick_worker(tty);
- n_tty_write_wakeup(tty->link);
- if (waitqueue_active(&tty->link->write_wait))
- wake_up_interruptible_poll(&tty->link->write_wait, POLLOUT);
+ tty_wakeup(tty->link);
return;
}
--
2.6.3
--
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