linux-kernel - Re: [linux-next] BUG triggered in ptraceme

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Date:   Wed, 19 Sep 2018 16:16:50 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, avagin@...tuozzo.com,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [linux-next] BUG triggered in ptraceme

Adding FS people to figure out whether GFP_KERNEL allocations with
i_rwsem's held for writing are okay.

On Wed, Sep 19, 2018 at 9:10 AM Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, Sep 19, 2018 at 10:07:37AM +0300, Cyrill Gorcunov wrote:
> > Hi Oleg! While been testing criu with linux-next we've triggered a BUG.
> > https://api.travis-ci.org/v3/job/430308998/log.txt
> >
> > [    2.461618] BUG: sleeping function called from invalid context at security/apparmor/include/cred.h:154
> > [    2.461794] in_atomic(): 1, irqs_disabled(): 1, pid: 152, name: init
> > [    2.461890] 1 lock held by init/152:
> > [    2.461981]  #0: 00000000f30c3fda (tasklist_lock){.+.+}, at: ptrace_traceme+0x1c/0x70
> > [    2.462114] irq event stamp: 2524
> > [    2.462242] hardirqs last  enabled at (2523): [<ffffffff98002922>] do_syscall_64+0x12/0x190
> > [    2.462363] hardirqs last disabled at (2524): [<ffffffff98b8b02f>] _raw_write_lock_irq+0xf/0x40
> > [    2.462476] softirqs last  enabled at (1904): [<ffffffff98ac79ef>] unix_sock_destructor+0x4f/0xc0
> > [    2.462586] softirqs last disabled at (1902): [<ffffffff98ac79ef>] unix_sock_destructor+0x4f/0xc0
> > [    2.462697] CPU: 1 PID: 152 Comm: init Not tainted 4.19.0-rc4-next-20180918+ #1
> >
> > Which is due to commit
> >
> > commit 4b105cbbaf7c06e01c27391957dc3c446328d087
> > Author: Oleg Nesterov <oleg@...hat.com>
> > Date:   Wed Jun 17 16:27:33 2009 -0700
> >
> >     ptrace: do not use task_lock() for attach
> >
> > because now after write_lock_irq(&tasklist_lock); apparmor calls for
> > traceme and
> >
> > static inline struct aa_label *begin_current_label_crit_section(void)
> > {
> >       struct aa_label *label = aa_current_raw_label();
> >
> > -->   might_sleep();
> >
> > Take a look please, once time permit.
>
> Heh, actually not :) It is due to commit
>
> commit 1f8266ff58840d698a1e96d2274189de1bdf7969
> Author: Jann Horn <jannh@...gle.com>
> Date:   Thu Sep 13 18:12:09 2018 +0200
>
> which introduced might_sleep. Seems it is bad idea to send bug report
> without having a cup of coffee at the morning :)

Yeah, I fixed one sleep-in-atomic bug and figured I'd throw a
might_sleep() in there for good measure... sigh.
I guess now I have to go through all the callers of
begin_current_label_crit_section() to see what else looks wrong...

apparmor_ptrace_traceme() is wrong, as reported...

apparmor_path_link() looks icky, but I'm not sure - from what I can
tell, it's called with an i_rwsem held for writing, and that probably
makes calling back into filesystem context from there a bad idea?
OTOH, it's just the i_rwsem of a newly-created path, so I don't know
whether that's actually an issue...

security_path_rename() is called with two i_rwsem's held, but again,
I'm not sure whether that's a problem.