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Date:   Wed, 20 Dec 2017 00:28:03 -0500
From:   Dave Jones <davej@...emonkey.org.uk>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: proc_flush_task oops

On Tue, Dec 19, 2017 at 07:54:24PM -0600, Eric W. Biederman wrote:

 > > *Scratches my head*  I am not seeing anything obvious.
 > 
 > Can you try this patch as you reproduce this issue?
 > 
 > diff --git a/kernel/pid.c b/kernel/pid.c
 > index b13b624e2c49..df9e5d4d8f83 100644
 > --- a/kernel/pid.c
 > +++ b/kernel/pid.c
 > @@ -210,6 +210,7 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 >                 goto out_unlock;
 >         for ( ; upid >= pid->numbers; --upid) {
 >                 /* Make the PID visible to find_pid_ns. */
 > +               WARN_ON(!upid->ns->proc_mnt);
 >                 idr_replace(&upid->ns->idr, pid, upid->nr);
 >                 upid->ns->pid_allocated++;
 >         }
 > 
 > 
 > If the warning triggers it means the bug is in alloc_pid and somehow
 > something has gotten past the is_child_reaper check.

You're onto something.

WARNING: CPU: 1 PID: 12020 at kernel/pid.c:213 alloc_pid+0x230/0x280
CPU: 1 PID: 12020 Comm: trinity-c29 Not tainted 4.15.0-rc4-think+ #3 
RIP: 0010:alloc_pid+0x230/0x280
RSP: 0018:ffffc90009977d48 EFLAGS: 00010046
RAX: 0000000000000030 RBX: ffff8804fb431280 RCX: 8f5c28f5c28f5c29
RDX: ffff88050a00de40 RSI: ffffffff82005218 RDI: ffff8804fc6aa9a8
RBP: ffff8804fb431270 R08: 0000000000000000 R09: 0000000000000001
R10: ffffc90009977cc0 R11: eab94e31da7171b7 R12: ffff8804fb431260
R13: ffff8804fb431240 R14: ffffffff82005200 R15: ffff8804fb431268
FS:  00007f49b9065700(0000) GS:ffff88050a000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f49b906a000 CR3: 00000004f7446001 CR4: 00000000001606e0
DR0: 00007f0b4c405000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:
 copy_process.part.41+0x14fa/0x1e30
 _do_fork+0xe7/0x720
 ? rcu_read_lock_sched_held+0x6c/0x80
 ? syscall_trace_enter+0x2d7/0x340
 do_syscall_64+0x60/0x210
 entry_SYSCALL64_slow_path+0x25/0x25

followed immediately by...

Oops: 0000 [#1] SMP
CPU: 1 PID: 12020 Comm: trinity-c29 Tainted: G        W        4.15.0-rc4-think+ #3 
RIP: 0010:proc_flush_task+0x8e/0x1b0
RSP: 0018:ffffc90009977c40 EFLAGS: 00010286
RAX: 0000000000000001 RBX: 0000000000000001 RCX: 00000000fffffffb
RDX: 0000000000000000 RSI: ffffc90009977c50 RDI: 0000000000000000
RBP: ffffc90009977c63 R08: 0000000000000000 R09: 0000000000000002
R10: ffffc90009977b70 R11: ffffc90009977c64 R12: 0000000000000004
R13: 0000000000000000 R14: 0000000000000004 R15: ffff8804fb431240
FS:  00007f49b9065700(0000) GS:ffff88050a000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 00000004f7446001 CR4: 00000000001606e0
DR0: 00007f0b4c405000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:
 ? release_task+0xaf/0x680
 release_task+0xd2/0x680
 ? wait_consider_task+0xb82/0xce0
 wait_consider_task+0xbe9/0xce0
 ? do_wait+0xe1/0x330
 do_wait+0x151/0x330
 kernel_wait4+0x8d/0x150
 ? task_stopped_code+0x50/0x50
 SYSC_wait4+0x95/0xa0
 ? rcu_read_lock_sched_held+0x6c/0x80
 ? syscall_trace_enter+0x2d7/0x340
 ? do_syscall_64+0x60/0x210
 do_syscall_64+0x60/0x210
 entry_SYSCALL64_slow_path+0x25/0x25

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