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use-after-free in n_tty_read

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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
-----------------------------------------------------------------------------

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.
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use-after-free in n_tty_read Dmitry Vyukov <dvyukov@google.com> - 2015-11-26 14:40 +0100
  Re: use-after-free in n_tty_read Peter Hurley <peter@hurleysoftware.com> - 2015-11-26 14:50 +0100

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