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Date:   Wed, 29 Nov 2017 11:37:04 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     syzbot 
        <bot+9abea25706ae35022385a41f61e579ed66e88a3f@...kaller.appspotmail.com>
Cc:     David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: KASAN: use-after-free Read in sock_release

(Cc'ing fs people...)

On Wed, Nov 29, 2017 at 12:33 AM, syzbot
<bot+9abea25706ae35022385a41f61e579ed66e88a3f@...kaller.appspotmail.com>
wrote:
> Hello,
>
> syzkaller hit the following crash on
> 1d3b78bbc6e983fabb3fbf91b76339bf66e4a12c
> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.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.
>
>
> device syz3 left promiscuous mode
> device syz3 entered promiscuous mode
> ==================================================================
> BUG: KASAN: use-after-free in sock_release+0x1c6/0x1e0 net/socket.c:601
> Read of size 8 at addr ffff8801c8dd1d10 by task syz-executor4/31085
>
> CPU: 0 PID: 31085 Comm: syz-executor4 Not tainted 4.14.0+ #129
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> 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_load8_noabort+0x14/0x20 mm/kasan/report.c:430
>  sock_release+0x1c6/0x1e0 net/socket.c:601
>  sock_close+0x16/0x20 net/socket.c:1125
>  __fput+0x333/0x7f0 fs/file_table.c:210
>  ____fput+0x15/0x20 fs/file_table.c:244
>  task_work_run+0x199/0x270 kernel/task_work.c:113
>  exit_task_work include/linux/task_work.h:22 [inline]
>  do_exit+0x9bb/0x1ae0 kernel/exit.c:865
>  do_group_exit+0x149/0x400 kernel/exit.c:968
>  get_signal+0x73f/0x16c0 kernel/signal.c:2335
>  do_signal+0x94/0x1ee0 arch/x86/kernel/signal.c:809
>  exit_to_usermode_loop+0x214/0x310 arch/x86/entry/common.c:158
>  prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
>  syscall_return_slowpath+0x490/0x550 arch/x86/entry/common.c:264
>  entry_SYSCALL_64_fastpath+0x94/0x96
> RIP: 0033:0x452879
> RSP: 002b:00007fb1c2435ce8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
> RAX: fffffffffffffe00 RBX: 0000000000758100 RCX: 0000000000452879
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000758100
> RBP: 0000000000758100 R08: 0000000000000304 R09: 00000000007580d8
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 0000000000a6f7ff R14: 00007fb1c24369c0 R15: 000000000000000e
>
> Allocated by task 31066:
>  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
>  kmem_cache_alloc_trace+0x136/0x750 mm/slab.c:3613
>  kmalloc include/linux/slab.h:499 [inline]
>  sock_alloc_inode+0xb4/0x300 net/socket.c:253
>  alloc_inode+0x65/0x180 fs/inode.c:208
>  new_inode_pseudo+0x69/0x190 fs/inode.c:890
>  sock_alloc+0x41/0x270 net/socket.c:565
>  __sock_create+0x148/0x850 net/socket.c:1225
>  sock_create net/socket.c:1301 [inline]
>  SYSC_socket net/socket.c:1331 [inline]
>  SyS_socket+0xeb/0x200 net/socket.c:1311
>  entry_SYSCALL_64_fastpath+0x1f/0x96
>
> Freed by task 3039:
>  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:3491 [inline]
>  kfree+0xca/0x250 mm/slab.c:3806
>  __rcu_reclaim kernel/rcu/rcu.h:190 [inline]
>  rcu_do_batch kernel/rcu/tree.c:2758 [inline]
>  invoke_rcu_callbacks kernel/rcu/tree.c:3012 [inline]
>  __rcu_process_callbacks kernel/rcu/tree.c:2979 [inline]
>  rcu_process_callbacks+0xe79/0x17d0 kernel/rcu/tree.c:2996
>  __do_softirq+0x29d/0xbb2 kernel/softirq.c:285

This looks more like a fs issue than network, my fs knowledge
is not good enough to justify why the hell the inode could be
destroyed before we release the fd.

My _guess_ is that it is because we defer the ____fput() to a
task work. If this is the case, then fs layer is not guilty for this.

On the other hand, if we have to blame net layer, it does look
suspicious on the RCU usage in sock_release() where we
claim RCU protection but I don't see we hold any RCU lock
there. Also, the code that deferences sock->wq is pretty much
useless now, at least I don't see it catches any bug though.


diff --git a/net/socket.c b/net/socket.c
index 42d8e9c9ccd5..b2390b5591a9 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -598,9 +598,6 @@ void sock_release(struct socket *sock)
                module_put(owner);
        }

-       if (rcu_dereference_protected(sock->wq, 1)->fasync_list)
-               pr_err("%s: fasync list not empty!\n", __func__);
-
        this_cpu_sub(sockets_in_use, 1);
        if (!sock->file) {
                iput(SOCK_INODE(sock));

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