[PATCH 5.15 491/917] nbd: Fix use-after-free in pid_show

Greg Kroah-Hartman
2021-11-16T00:49:56Z

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 1183f7872b713..4f1b591c3a555 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -122,10 +122,10 @@ struct nbd_device {
struct work_struct remove_work;

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

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

char *backend;
};
@@ -217,7 +217,7 @@ 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 = {
@@ -329,7 +329,7 @@ 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) {
@@ -1241,7 +1241,7 @@ 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);
@@ -1282,7 +1282,7 @@ 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;
@@ -1301,7 +1301,7 @@ 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);

@@ -1557,8 +1557,8 @@ 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;
}
@@ -2135,7 +2135,7 @@ 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