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    Subject[PATCH 5.14 483/849] nbd: Fix use-after-free in pid_show
    Date
    From: Ye Bin <yebin10@huawei.com>

    [ Upstream commit 0c98057be9efa32de78dbc4685fc73da9d71faa1 ]

    I got issue as follows:
    [ 263.886511] BUG: KASAN: use-after-free in pid_show+0x11f/0x13f
    [ 263.888359] Read of size 4 at addr ffff8880bf0648c0 by task cat/746
    [ 263.890479] CPU: 0 PID: 746 Comm: cat Not tainted 4.19.90-dirty #140
    [ 263.893162] Call Trace:
    [ 263.893509] dump_stack+0x108/0x15f
    [ 263.893999] print_address_description+0xa5/0x372
    [ 263.894641] kasan_report.cold+0x236/0x2a8
    [ 263.895696] __asan_report_load4_noabort+0x25/0x30
    [ 263.896365] pid_show+0x11f/0x13f
    [ 263.897422] dev_attr_show+0x48/0x90
    [ 263.898361] sysfs_kf_seq_show+0x24d/0x4b0
    [ 263.899479] kernfs_seq_show+0x14e/0x1b0
    [ 263.900029] seq_read+0x43f/0x1150
    [ 263.900499] kernfs_fop_read+0xc7/0x5a0
    [ 263.903764] vfs_read+0x113/0x350
    [ 263.904231] ksys_read+0x103/0x270
    [ 263.905230] __x64_sys_read+0x77/0xc0
    [ 263.906284] do_syscall_64+0x106/0x360
    [ 263.906797] entry_SYSCALL_64_after_hwframe+0x44/0xa9

    Reproduce this issue as follows:
    1. nbd-server 8000 /tmp/disk
    2. nbd-client localhost 8000 /dev/nbd1
    3. cat /sys/block/nbd1/pid
    Then trigger use-after-free in pid_show.

    Reason is after do step '2', nbd-client progress is already exit. So
    it's task_struct already freed.
    To solve this issue, revert part of 6521d39a64b3's modify and remove
    useless 'recv_task' member of nbd_device.

    Fixes: 6521d39a64b3 ("nbd: Remove variable 'pid'")
    Signed-off-by: Ye Bin <yebin10@huawei.com>
    Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Link: https://lore.kernel.org/r/20211020073959.2679255-1-yebin10@huawei.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/block/nbd.c | 18 +++++++++---------
    1 file changed, 9 insertions(+), 9 deletions(-)

    diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
    index 99ab58b877f8c..6d7e181d3ed1f 100644
    --- a/drivers/block/nbd.c
    +++ b/drivers/block/nbd.c
    @@ -122,11 +122,11 @@ struct nbd_device {
    struct work_struct remove_work;

    struct list_head list;
    - struct task_struct *task_recv;
    struct task_struct *task_setup;

    struct completion *destroy_complete;
    unsigned long flags;
    + pid_t pid; /* pid of nbd-client, if attached */

    char *backend;
    };
    @@ -218,7 +218,7 @@ static ssize_t pid_show(struct device *dev,
    struct gendisk *disk = dev_to_disk(dev);
    struct nbd_device *nbd = (struct nbd_device *)disk->private_data;

    - return sprintf(buf, "%d\n", task_pid_nr(nbd->task_recv));
    + return sprintf(buf, "%d\n", nbd->pid);
    }

    static const struct device_attribute pid_attr = {
    @@ -362,7 +362,7 @@ static int nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
    nbd->config->bytesize = bytesize;
    nbd->config->blksize_bits = __ffs(blksize);

    - if (!nbd->task_recv)
    + if (!nbd->pid)
    return 0;

    if (nbd->config->flags & NBD_FLAG_SEND_TRIM) {
    @@ -1274,7 +1274,7 @@ static void nbd_config_put(struct nbd_device *nbd)
    if (test_and_clear_bit(NBD_RT_HAS_PID_FILE,
    &config->runtime_flags))
    device_remove_file(disk_to_dev(nbd->disk), &pid_attr);
    - nbd->task_recv = NULL;
    + nbd->pid = 0;
    if (test_and_clear_bit(NBD_RT_HAS_BACKEND_FILE,
    &config->runtime_flags)) {
    device_remove_file(disk_to_dev(nbd->disk), &backend_attr);
    @@ -1315,7 +1315,7 @@ static int nbd_start_device(struct nbd_device *nbd)
    int num_connections = config->num_connections;
    int error = 0, i;

    - if (nbd->task_recv)
    + if (nbd->pid)
    return -EBUSY;
    if (!config->socks)
    return -EINVAL;
    @@ -1334,7 +1334,7 @@ static int nbd_start_device(struct nbd_device *nbd)
    }

    blk_mq_update_nr_hw_queues(&nbd->tag_set, config->num_connections);
    - nbd->task_recv = current;
    + nbd->pid = task_pid_nr(current);

    nbd_parse_flags(nbd);

    @@ -1590,8 +1590,8 @@ static int nbd_dbg_tasks_show(struct seq_file *s, void *unused)
    {
    struct nbd_device *nbd = s->private;

    - if (nbd->task_recv)
    - seq_printf(s, "recv: %d\n", task_pid_nr(nbd->task_recv));
    + if (nbd->pid)
    + seq_printf(s, "recv: %d\n", nbd->pid);

    return 0;
    }
    @@ -2177,7 +2177,7 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
    mutex_lock(&nbd->config_lock);
    config = nbd->config;
    if (!test_bit(NBD_RT_BOUND, &config->runtime_flags) ||
    - !nbd->task_recv) {
    + !nbd->pid) {
    dev_err(nbd_to_dev(nbd),
    "not configured, cannot reconfigure\n");
    ret = -EINVAL;
    --
    2.33.0


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