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Date:   Wed, 01 Mar 2017 23:09:05 -0800
From:   "Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:     "Bryant G. Ly" <bryantly@...ux.vnet.ibm.com>
Cc:     target-devel <target-devel@...r.kernel.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Rob Millner <rlm@...erainc.com>, Vaibhav Tandon <vst@...era.io>
Subject: Re: [PATCH] target: Fix NULL dereference during LUN lookup + active
 I/O shutdown

On Thu, 2017-02-23 at 11:46 -0600, Bryant G. Ly wrote:
> > From: Nicholas Bellinger <nab@...ux-iscsi.org>
> >
> > When transport_clear_lun_ref() is shutting down a se_lun via
> > configfs with new I/O in-flight, it's possible to trigger a
> > NULL pointer dereference in transport_lookup_cmd_lun() due
> > to the fact percpu_ref_get() doesn't do any __PERCPU_REF_DEAD
> > checking before incrementing lun->lun_ref.count after
> > lun->lun_ref has switched to atomic_t mode.
> >
> > This results in a NULL pointer dereference as LUN shutdown
> > code in core_tpg_remove_lun() continues running after the
> > existing ->release() -> core_tpg_lun_ref_release() callback
> > completes, and clears the RCU protected se_lun->lun_se_dev
> > pointer.
> >
> > During the OOPs, the state of lun->lun_ref in the process
> > which triggered the NULL pointer dereference looks like
> > the following on v4.1.y stable code:
> >
> > struct se_lun {
> >    lun_link_magic = 4294932337,
> >    lun_status = TRANSPORT_LUN_STATUS_FREE,
> >
> >    .....
> >
> >    lun_se_dev = 0x0,
> >    lun_sep = 0x0,
> >
> >    .....
> >
> >    lun_ref = {
> >      count = {
> >        counter = 1
> >      },
> >      percpu_count_ptr = 3,
> >      release = 0xffffffffa02fa1e0 <core_tpg_lun_ref_release>,
> >      confirm_switch = 0x0,
> >      force_atomic = false,
> >      rcu = {
> >        next = 0xffff88154fa1a5d0,
> >        func = 0xffffffff8137c4c0 <percpu_ref_switch_to_atomic_rcu>
> >      }
> >    }
> > }
> >
> > To address this bug, use percpu_ref_tryget_live() to ensure
> > once __PERCPU_REF_DEAD is visable on all CPUs and ->lun_ref
> > has switched to atomic_t, all new I/Os will fail to obtain
> > a new lun->lun_ref reference.
> >
> > Also use an explicit percpu_ref_kill_and_confirm() callback
> > to block on ->lun_ref_comp to allow the first stage and
> > associated RCU grace period to complete, and then block on
> > ->lun_ref_shutdown waiting for the final percpu_ref_put()
> > to drop the last reference via transport_lun_remove_cmd()
> > before continuing with core_tpg_remove_lun() shutdown.
> >
> > Reported-by: Rob Millner <rlm@...erainc.com>
> > Tested-by: Rob Millner <rlm@...erainc.com>
> > Cc: Rob Millner <rlm@...erainc.com>
> > Tested-by: Vaibhav Tandon <vst@...era.io>
> > Cc: Vaibhav Tandon <vst@...era.io>
> > Signed-off-by: Nicholas Bellinger <nab@...ux-iscsi.org>
> > ---
> >   drivers/target/target_core_device.c    | 10 ++++++++--
> >   drivers/target/target_core_tpg.c       |  3 ++-
> >   drivers/target/target_core_transport.c | 31 ++++++++++++++++++++++++++++++-
> >   include/target/target_core_base.h      |  1 +
> >   4 files changed, 41 insertions(+), 4 deletions(-)
> >
> I have seen this and have tested this with our custom kernel.
> 
> So this looks good from me!
> 

Added your Tested-by to the patch.

Thanks Bryant.

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