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Subject: Re: [PATCH v2] security: device_cgroup: fix RCU lockdep splat
From: Josh Boyer <jwboyer@fedoraproject.org>
To: Felipe Balbi <balbi@ti.com>
Cc: James Morris <james.l.morris@oracle.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
        Linux OMAP Mailing List <linux-omap@vger.kernel.org>
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On Wed, Sep 2, 2015 at 9:12 AM, Felipe Balbi <balbi@ti.com> wrote:
> while booting AM437x device, the following splat
> triggered:
>
> [   12.005238] ===============================
> [   12.009749] [ INFO: suspicious RCU usage. ]
> [   12.014116] 4.2.0-next-20150831 #1154 Not tainted
> [   12.019050] -------------------------------
> [   12.023408] security/device_cgroup.c:405 device_cgroup:verify_new_ex called without proper synchronization!
> [   12.033576] other info that might help us debug this:
>
> [   12.041942] rcu_scheduler_active = 1, debug_locks = 0
> [   12.048796] 4 locks held by systemd/1:
> [   12.052700]  #0:  (sb_writers#7){.+.+.+}, at: [<c017af84>] __sb_start_write+0x8c/0xb0
> [   12.060954]  #1:  (&of->mutex){+.+.+.}, at: [<c01f1600>] kernfs_fop_write+0x50/0x1b8
> [   12.069085]  #2:  (s_active#30){++++.+}, at: [<c01f1608>] kernfs_fop_write+0x58/0x1b8
> [   12.077310]  #3:  (devcgroup_mutex){+.+...}, at: [<c0317bfc>] devcgroup_access_write+0x20/0x658
> [   12.086575] stack backtrace:
> [   12.091124] CPU: 0 PID: 1 Comm: systemd Not tainted 4.2.0-next-20150831 #1154
> [   12.098609] Hardware name: Generic AM43 (Flattened Device Tree)
> [   12.104807] [<c001770c>] (unwind_backtrace) from [<c0013a58>] (show_stack+0x10/0x14)
> [   12.112924] [<c0013a58>] (show_stack) from [<c034f014>] (dump_stack+0x84/0x9c)
> [   12.120491] [<c034f014>] (dump_stack) from [<c0317a04>] (verify_new_ex+0xc4/0xdc)
> [   12.128326] [<c0317a04>] (verify_new_ex) from [<c0317f50>] (devcgroup_access_write+0x374/0x658)
> [   12.137426] [<c0317f50>] (devcgroup_access_write) from [<c00d2800>] (cgroup_file_write+0x28/0x1bc)
> [   12.146796] [<c00d2800>] (cgroup_file_write) from [<c01f1670>] (kernfs_fop_write+0xc0/0x1b8)
> [   12.155620] [<c01f1670>] (kernfs_fop_write) from [<c0177c94>] (__vfs_write+0x1c/0xd8)
> [   12.163783] [<c0177c94>] (__vfs_write) from [<c0178594>] (vfs_write+0x90/0x16c)
> [   12.171426] [<c0178594>] (vfs_write) from [<c0178db4>] (SyS_write+0x44/0x9c)
> [   12.178806] [<c0178db4>] (SyS_write) from [<c000f680>] (ret_fast_syscall+0x0/0x1c)
>
> Fix it by making sure rcu_read_lock() is held
> around calls to parent_has_perm().
>
> Signed-off-by: Felipe Balbi <balbi@ti.com>

This cleared up the splat on all my machines and I don't see any other
side effects (even with lockdep enabled).  Thanks!

Tested-by: Josh Boyer <jwboyer@fedoraproject.org>

josh

> ---
>
> Changes since v1:
>         - move rcu_read_lock/unlock to wrap parent_has_perm()
>
>  security/device_cgroup.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 73455089feef..dd77ed206fa4 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -608,6 +608,7 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
>         int count, rc = 0;
>         struct dev_exception_item ex;
>         struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
> +       int ret;
>
>         if (!capable(CAP_SYS_ADMIN))
>                 return -EPERM;
> @@ -734,7 +735,11 @@ static int devcgroup_update_access(struct dev_cgroup *devcgroup,
>                         break;
>                 }
>
> -               if (!parent_has_perm(devcgroup, &ex))
> +               rcu_read_lock();
> +               ret = parent_has_perm(devcgroup, &ex);
> +               rcu_read_unlock();
> +
> +               if (!ret)
>                         return -EPERM;
>                 rc = dev_exception_add(devcgroup, &ex);
>                 break;
> --
> 2.5.0
>
> --
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