Re: [PATCH] target: Fix NULL dereference during LUN lookup + active I/O shutdown

From: Bryant G. Ly
Date: Thu Feb 23 2017 - 12:48:36 EST

From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>

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

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_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@xxxxxxxxxxxxx>
Tested-by: Rob Millner <rlm@xxxxxxxxxxxxx>
Cc: Rob Millner <rlm@xxxxxxxxxxxxx>
Tested-by: Vaibhav Tandon <vst@xxxxxxxxx>
Cc: Vaibhav Tandon <vst@xxxxxxxxx>
Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
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!