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    Subject[PATCH-next] scsi: fix use-after-free problem in scsi_remove_target
    Date
    From: Zhong Jinghua <zhongjinghua@huawei.com>

    A use-after-free problem like below:

    BUG: KASAN: use-after-free in scsi_target_reap+0x6c/0x70

    Workqueue: scsi_wq_1 __iscsi_unbind_session [scsi_transport_iscsi]
    Call trace:
    dump_backtrace+0x0/0x320
    show_stack+0x24/0x30
    dump_stack+0xdc/0x128
    print_address_description+0x68/0x278
    kasan_report+0x1e4/0x308
    __asan_report_load4_noabort+0x30/0x40
    scsi_target_reap+0x6c/0x70
    scsi_remove_target+0x430/0x640
    __iscsi_unbind_session+0x164/0x268 [scsi_transport_iscsi]
    process_one_work+0x67c/0x1350
    worker_thread+0x370/0xf90
    kthread+0x2a4/0x320
    ret_from_fork+0x10/0x18

    The problem is caused by a concurrency scenario:

    T0: delete target
    // echo 1 > /sys/devices/platform/host1/session1/target1:0:0/1:0:0:1/delete
    T1: logout
    // iscsiadm -m node --logout

    T0 T1
    sdev_store_delete
    scsi_remove_device
    device_remove_file
    __scsi_remove_device
    __iscsi_unbind_session
    scsi_remove_target
    spin_lock_irqsave
    list_for_each_entry
    scsi_target_reap // starget->reaf 1 -> 0
    kref_get(&starget->reap_ref);
    // warn use-after-free.
    spin_unlock_irqrestore
    scsi_target_reap_ref_release
    scsi_target_destroy
    ... // delete starget
    scsi_target_reap
    // UAF

    When T0 reduces the reference count to 0, but has not been released,
    T1 can still enter list_for_each_entry, and then kref_get reports UAF.

    Fix it by using kref_get_unless_zero() to check for a reference count of
    0.

    Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
    ---
    drivers/scsi/scsi_sysfs.c | 12 +++++++++++-
    1 file changed, 11 insertions(+), 1 deletion(-)

    diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
    index e7893835b99a..0ad357ff4c59 100644
    --- a/drivers/scsi/scsi_sysfs.c
    +++ b/drivers/scsi/scsi_sysfs.c
    @@ -1561,7 +1561,17 @@ void scsi_remove_target(struct device *dev)
    starget->state == STARGET_CREATED_REMOVE)
    continue;
    if (starget->dev.parent == dev || &starget->dev == dev) {
    - kref_get(&starget->reap_ref);
    +
    + /*
    + * If starget->reap_ref is reduced to 0, it means
    + * that other processes are releasing it and
    + * there is no need to delete it again
    + */
    + if (!kref_get_unless_zero(&starget->reap_ref)) {
    + spin_unlock_irqrestore(shost->host_lock, flags);
    + goto restart;
    + }
    +
    if (starget->state == STARGET_CREATED)
    starget->state = STARGET_CREATED_REMOVE;
    else
    --
    2.31.1
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