Subject: Re: KASAN: use-after-free Read in inet_create

From: Eric Biggers <[email-masked]>
Date: Sun, 8 Apr 2018 16:17:56 -0700

[+RDS list and maintainer]

On Sat, Dec 09, 2017 at 12:50:01PM -0800, syzbot wrote:
> Hello,
> 
> syzkaller hit the following crash on
> 82bcf1def3b5f1251177ad47c44f7e17af039b4b
> git://git.cmpxchg.org/linux-mmots.git/master
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached
> Raw console output is attached.
> 
> Unfortunately, I don't have any reproducer for this bug yet.
> 
> 
> ==================================================================
> BUG: KASAN: use-after-free in inet_create+0xda0/0xf50 net/ipv4/af_inet.c:338
> Read of size 4 at addr ffff8801bde28554 by task kworker/u4:5/3492
> 
> CPU: 0 PID: 3492 Comm: kworker/u4:5 Not tainted 4.15.0-rc2-mm1+ #39
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Workqueue: krdsd rds_connect_worker
> Call Trace:
>  __dump_stack lib/dump_stack.c:17 [inline]
>  dump_stack+0x194/0x257 lib/dump_stack.c:53
>  print_address_description+0x73/0x250 mm/kasan/report.c:252
>  kasan_report_error mm/kasan/report.c:351 [inline]
>  kasan_report+0x25b/0x340 mm/kasan/report.c:409
>  __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:429
>  inet_create+0xda0/0xf50 net/ipv4/af_inet.c:338
>  __sock_create+0x4d4/0x850 net/socket.c:1265
>  sock_create_kern+0x3f/0x50 net/socket.c:1311
>  rds_tcp_conn_path_connect+0x26f/0x920 net/rds/tcp_connect.c:108
>  rds_connect_worker+0x156/0x1f0 net/rds/threads.c:165
>  process_one_work+0xbfd/0x1bc0 kernel/workqueue.c:2113
>  worker_thread+0x223/0x1990 kernel/workqueue.c:2247
>  kthread+0x37a/0x440 kernel/kthread.c:238
>  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:524
> 
> Allocated by task 3362:
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>  set_track mm/kasan/kasan.c:459 [inline]
>  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
>  kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
>  kmem_cache_alloc+0x12e/0x760 mm/slab.c:3548
>  kmem_cache_zalloc include/linux/slab.h:695 [inline]
>  net_alloc net/core/net_namespace.c:362 [inline]
>  copy_net_ns+0x196/0x580 net/core/net_namespace.c:402
>  create_new_namespaces+0x425/0x880 kernel/nsproxy.c:107
>  unshare_nsproxy_namespaces+0xae/0x1e0 kernel/nsproxy.c:206
>  SYSC_unshare kernel/fork.c:2421 [inline]
>  SyS_unshare+0x653/0xfa0 kernel/fork.c:2371
>  entry_SYSCALL_64_fastpath+0x1f/0x96
> 
> Freed by task 35:
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>  set_track mm/kasan/kasan.c:459 [inline]
>  kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
>  __cache_free mm/slab.c:3492 [inline]
>  kmem_cache_free+0x77/0x280 mm/slab.c:3750
>  net_free+0xca/0x110 net/core/net_namespace.c:378
>  net_drop_ns.part.11+0x26/0x30 net/core/net_namespace.c:385
>  net_drop_ns net/core/net_namespace.c:384 [inline]
>  cleanup_net+0x895/0xb60 net/core/net_namespace.c:502
>  process_one_work+0xbfd/0x1bc0 kernel/workqueue.c:2113
>  worker_thread+0x223/0x1990 kernel/workqueue.c:2247
>  kthread+0x37a/0x440 kernel/kthread.c:238
>  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:524
> 
> The buggy address belongs to the object at ffff8801bde28080
>  which belongs to the cache net_namespace of size 6272
> The buggy address is located 1236 bytes inside of
>  6272-byte region [ffff8801bde28080, ffff8801bde29900)
> The buggy address belongs to the page:
> page:00000000df6a4dc0 count:1 mapcount:0 mapping:00000000553659f1 index:0x0
> compound_mapcount: 0
> flags: 0x2fffc0000008100(slab|head)
> raw: 02fffc0000008100 ffff8801bde28080 0000000000000000 0000000100000001
> raw: ffffea0006f75da0 ffffea0006f60220 ffff8801d989fe00 0000000000000000
> page dumped because: kasan: bad access detected
> 
> Memory state around the buggy address:
>  ffff8801bde28400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff8801bde28480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > ffff8801bde28500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                                                  ^
>  ffff8801bde28580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff8801bde28600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> 
> ---
> This bug is generated by a dumb bot. It may contain errors.
> See https://goo.gl/tpsmEJ for details.
> Direct all questions to [email-masked].
> Please credit me with: Reported-by: syzbot <[email-masked]>
> 
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> 

This is still happening regularly, though syzbot hasn't been able to generate a
reproducer yet.  All the reports seem to involve rds_connect_worker()
encountering a freed network namespace (struct net) when calling
sock_create_kern() from rds_tcp_conn_path_connect().  Probably something in RDS
needs to be taking a reference to the network namespace and isn't, or the RDS
workqueue isn't being shut down correctly.  You can see all reports of this on
the syzbot dashboard at
https://syzkaller.appspot.com/bug?id=1f45ae538a0453220337ccb84962249fdd67107f.
Last one was April 5 on Linus' tree (commit 3e968c9f1401088).

- Eric