Re: [PATCH -next] nbd: Fix use-after-free in pid_show

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Oct 20, 2021 at 03:39:59PM +0800, Ye Bin wrote:
> 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>

Thanks,

Josef