Date:   Sun, 1 Apr 2018 13:30:23 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs@...glegroups.com,
        syzbot+84371b6062cb639d797e@...kaller.appspotmail.com
Subject: Re: WARNING: refcount bug in should_fail

On Sun, Apr 1, 2018 at 1:11 PM, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> Dmitry Vyukov wrote:
>> On Sun, Apr 1, 2018 at 12:32 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> > On Sun, Mar 4, 2018 at 6:57 AM, Tetsuo Handa
>> > <penguin-kernel@...ove.sakura.ne.jp> wrote:
>> >> Switching from mm to fsdevel, for this report says that put_net(net) in
>> >> rpc_kill_sb() made net->count < 0 when mount_ns() failed due to
>> >> register_shrinker() failure.
>> >>
>> >> Relevant commits will be
>> >> commit 9ee332d99e4d5a97 ("sget(): handle failures of register_shrinker()") and
>> >> commit d91ee87d8d85a080 ("vfs: Pass data, ns, and ns->userns to mount_ns.").
>> >>
>> >> When sget_userns() in mount_ns() failed, mount_ns() returns an error code to
>> >> the caller without calling fill_super(). That is, get_net(sb->s_fs_info) was
>> >> not called by rpc_fill_super() (via fill_super callback passed to mount_ns())
>> >> but put_net(sb->s_fs_info) is called by rpc_kill_sb() (via fs->kill_sb() from
>> >> deactivate_locked_super()).
>> >>
>> >> ----------
>> >> static struct dentry *
>> >> rpc_mount(struct file_system_type *fs_type,
>> >>                 int flags, const char *dev_name, void *data)
>> >> {
>> >>         struct net *net = current->nsproxy->net_ns;
>> >>         return mount_ns(fs_type, flags, data, net, net->user_ns, rpc_fill_super);
>> >> }
>> >> ----------
>> >
>> > Messed kernel output, this is definitely not in should_fail.
>> >
>> > #syz dup: WARNING: refcount bug in sk_alloc
>>
>> Please don't drop reporter (syzbot) email from CC.
>>
>> #syz dup: WARNING: refcount bug in sk_alloc
>>
>
> Excuse me? This "refcount bug in should_fail" is talking about sget_userns() versus rpc_fill_super().
> I think we need to fix either 9ee332d99e4d5a97 or d91ee87d8d85a080.

Hi,

I think I was looking at this incarnation of this bug before marking it as dup:
https://syzkaller.appspot.com/text?tag=CrashReport&id=5246446760624128
that report in fact includes sk_alloc frame. Kernel turning crash
reports into untangleable mess is not really helpful.

I will undup this into an independent bug.
It's just that we don't have such functionality yet, so I need to
implement it first.





>> >> syzbot wrote:
>> >>> Hello,
>> >>>
>> >>> syzbot hit the following crash on bpf-next commit
>> >>> 6f1b5a2b58d8470e5a8b25ab29f5fdb4616ffff8 (Tue Feb 27 04:11:23 2018 +0000)
>> >>> Merge branch 'bpf-kselftest-improvements'
>> >>>
>> >>> C reproducer is attached.
>> >>> syzkaller reproducer is attached.
>> >>> Raw console output is attached.
>> >>> compiler: gcc (GCC) 7.1.1 20170620
>> >>> .config is attached.
>> >>>
>> >>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> >>> Reported-by: syzbot+84371b6062cb639d797e@...kaller.appspotmail.com
>> >>> It will help syzbot understand when the bug is fixed. See footer for
>> >>> details.
>> >>> If you forward the report, please keep this part and the footer.
>> >>>
>> >>> ------------[ cut here ]------------
>> >>> FAULT_INJECTION: forcing a failure.
>> >>> name failslab, interval 1, probability 0, space 0, times 0
>> >>> refcount_t: underflow; use-after-free.
>> >>> CPU: 1 PID: 4239 Comm: syzkaller149381 Not tainted 4.16.0-rc2+ #20
>> >>> WARNING: CPU: 0 PID: 4237 at lib/refcount.c:187
>> >>> refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:187
>> >>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> >>> Google 01/01/2011
>> >>> Call Trace:
>> >>> Kernel panic - not syncing: panic_on_warn set ...
>> >>>
>> >>>   __dump_stack lib/dump_stack.c:17 [inline]
>> >>>   dump_stack+0x194/0x24d lib/dump_stack.c:53
>> >>>   fail_dump lib/fault-inject.c:51 [inline]
>> >>>   should_fail+0x8c0/0xa40 lib/fault-inject.c:149
>> >>>   should_failslab+0xec/0x120 mm/failslab.c:32
>> >>>   slab_pre_alloc_hook mm/slab.h:422 [inline]
>> >>>   slab_alloc mm/slab.c:3365 [inline]
>> >>>   __do_kmalloc mm/slab.c:3703 [inline]
>> >>>   __kmalloc+0x63/0x760 mm/slab.c:3714
>> >>>   kmalloc include/linux/slab.h:517 [inline]
>> >>>   kzalloc include/linux/slab.h:701 [inline]
>> >>>   register_shrinker+0x10e/0x2d0 mm/vmscan.c:268
>> >>>   sget_userns+0xbbf/0xe40 fs/super.c:520
>> >>>   mount_ns+0x6d/0x190 fs/super.c:1029
>> >>>   rpc_mount+0x9e/0xd0 net/sunrpc/rpc_pipe.c:1451
>> >>>   mount_fs+0x66/0x2d0 fs/super.c:1222
>> >>>   vfs_kern_mount.part.26+0xc6/0x4a0 fs/namespace.c:1037
>> >>>   vfs_kern_mount fs/namespace.c:2509 [inline]
>> >>>   do_new_mount fs/namespace.c:2512 [inline]
>> >>>   do_mount+0xea4/0x2bb0 fs/namespace.c:2842
>> >>>   SYSC_mount fs/namespace.c:3058 [inline]
>> >>>   SyS_mount+0xab/0x120 fs/namespace.c:3035
>> >>>   do_syscall_64+0x280/0x940 arch/x86/entry/common.c:287
>> >>>   entry_SYSCALL_64_after_hwframe+0x42/0xb7
>> >>> RIP: 0033:0x4460f9
>> >>> RSP: 002b:00007fbcd769ad78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
>> >>> RAX: ffffffffffffffda RBX: 00000000006dcc6c RCX: 00000000004460f9
>> >>> RDX: 0000000020000080 RSI: 0000000020000040 RDI: 0000000020000000
>> >>> RBP: 00007fbcd769ad80 R08: 00000000200000c0 R09: 0000000000003131
>> >>> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc68
>> >>> R13: ffffffffffffffff R14: 0000000000000037 R15: 0030656c69662f2e
>> >>> CPU: 0 PID: 4237 Comm: syzkaller149381 Not tainted 4.16.0-rc2+ #20
>> >>> 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+0x194/0x24d lib/dump_stack.c:53
>> >>>   panic+0x1e4/0x41c kernel/panic.c:183
>> >>>   __warn+0x1dc/0x200 kernel/panic.c:547
>> >>>   report_bug+0x211/0x2d0 lib/bug.c:184
>> >>>   fixup_bug.part.11+0x37/0x80 arch/x86/kernel/traps.c:178
>> >>>   fixup_bug arch/x86/kernel/traps.c:247 [inline]
>> >>>   do_error_trap+0x2d7/0x3e0 arch/x86/kernel/traps.c:296
>> >>>   do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:315
>> >>>   invalid_op+0x58/0x80 arch/x86/entry/entry_64.S:957
>> >>> RIP: 0010:refcount_sub_and_test+0x167/0x1b0 lib/refcount.c:187
>> >>> RSP: 0018:ffff8801b164f6d8 EFLAGS: 00010286
>> >>> RAX: dffffc0000000008 RBX: 0000000000000000 RCX: ffffffff815ac30e
>> >>> RDX: 0000000000000000 RSI: 1ffff100362c9e8b RDI: 1ffff100362c9e60
>> >>> RBP: ffff8801b164f768 R08: 0000000000000000 R09: 0000000000000000
>> >>> R10: ffff8801b164f610 R11: 0000000000000000 R12: 1ffff100362c9edc
>> >>> R13: 00000000ffffffff R14: 0000000000000001 R15: ffff8801ae924044
>> >>>   refcount_dec_and_test+0x1a/0x20 lib/refcount.c:212
>> >>>   put_net include/net/net_namespace.h:220 [inline]
>> >>>   rpc_kill_sb+0x253/0x3c0 net/sunrpc/rpc_pipe.c:1473
>> >>>   deactivate_locked_super+0x88/0xd0 fs/super.c:312
>> >>>   sget_userns+0xbda/0xe40 fs/super.c:522
>> >>>   mount_ns+0x6d/0x190 fs/super.c:1029
>> >>>   rpc_mount+0x9e/0xd0 net/sunrpc/rpc_pipe.c:1451
>> >>>   mount_fs+0x66/0x2d0 fs/super.c:1222
>> >>>   vfs_kern_mount.part.26+0xc6/0x4a0 fs/namespace.c:1037
>> >>>   vfs_kern_mount fs/namespace.c:2509 [inline]
>> >>>   do_new_mount fs/namespace.c:2512 [inline]
>> >>>   do_mount+0xea4/0x2bb0 fs/namespace.c:2842
>> >>>   SYSC_mount fs/namespace.c:3058 [inline]
>> >>>   SyS_mount+0xab/0x120 fs/namespace.c:3035
>> >>>   do_syscall_64+0x280/0x940 arch/x86/entry/common.c:287
>> >>>   entry_SYSCALL_64_after_hwframe+0x42/0xb7
>> >>> RIP: 0033:0x4460f9
>> >>> RSP: 002b:00007fbcd76dcd78 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
>> >>> RAX: ffffffffffffffda RBX: 00000000006dcc3c RCX: 00000000004460f9
>> >>> RDX: 0000000020000080 RSI: 0000000020000040 RDI: 0000000020000000
>> >>> RBP: 00007fbcd76dcd80 R08: 00000000200000c0 R09: 0000000000003131
>> >>> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dcc38
>> >>> R13: ffffffffffffffff R14: 0000000000000028 R15: 0030656c69662f2e
>> >>> Dumping ftrace buffer:
>> >>>     (ftrace buffer empty)
>> >>> Kernel Offset: disabled
>> >>> Rebooting in 86400 seconds..
>> >>>
>> >>>
>> >>> ---
>> >>> This bug is generated by a dumb bot. It may contain errors.
>> >>> See https://goo.gl/tpsmEJ for details.
>> >>> Direct all questions to syzkaller@...glegroups.com.
>> >>>
>> >>> syzbot will keep track of this bug report.
>> >>> If you forgot to add the Reported-by tag, once the fix for this bug is
>> >>> merged
>> >>> into any tree, please reply to this email with:
>> >>> #syz fix: exact-commit-title
>> >>> If you want to test a patch for this bug, please reply with:
>> >>> #syz test: git://repo/address.git branch
>> >>> and provide the patch inline or as an attachment.
>> >>> To mark this as a duplicate of another syzbot report, please reply with:
>> >>> #syz dup: exact-subject-of-another-report
>> >>> If it's a one-off invalid bug report, please reply with:
>> >>> #syz invalid
>> >>> Note: if the crash happens again, it will cause creation of a new bug
>> >>> report.
>> >>> Note: all commands must start from beginning of the line in the email body.
>> >>>
>> >>
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