Re: [PATCH] zram: Fix unbalanced idr management at hot removal

[Takashi Iwai]

On Mon, 21 Nov 2016 14:21:40 +0100,
Takashi Iwai wrote:
> 
> The zram hot removal code calls idr_remove() even when zram_remove()
> returns an error (typically -EBUSY).  This results in a leftover at
> the device release, eventually leading to a crash when the module is
> reloaded.
> 
> As described in the bug report below, the following procedure would
> cause an Oops with zram:
> 
> - provision three zram devices via modprobe zram num_devices=3
> - configure a size for each device
>   + echo "1G" > /sys/block/$zram_name/disksize
> - mkfs and mount zram0 only
> - attempt to hot remove all three devices
>   + echo 2 > /sys/class/zram-control/hot_remove
>   + echo 1 > /sys/class/zram-control/hot_remove
>   + echo 0 > /sys/class/zram-control/hot_remove
>      - zram0 removal fails with EBUSY, as expected
> - unmount zram0
> - try zram0 hot remove again
>   + echo 0 > /sys/class/zram-control/hot_remove
>      - fails with ENODEV (unexpected)
> - unload zram kernel module
>   + completes successfully
> - zram0 device node still exists
> - attempt to mount /dev/zram0
>   + mount command is killed
>   + following BUG is encountered
> 
>  BUG: unable to handle kernel paging request at ffffffffa0002ba0
>  IP: [<ffffffff812eead6>] get_disk+0x16/0x50
>  Oops: 0000 [#1] SMP
>  CPU: 0 PID: 252 Comm: mount Not tainted 4.9.0-rc6 #176
>  task: ffff88001a9f2800 task.stack: ffffc90000300000
>  RIP: 0010:[<ffffffff812eead6>]  [<ffffffff812eead6>] get_disk+0x16/0x50
>  Call Trace:
>   [<ffffffff812eeb1c>] exact_lock+0xc/0x20
>   [<ffffffff813b3e1c>] kobj_lookup+0xdc/0x160
>   [<ffffffff812edce0>] ? disk_map_sector_rcu+0x70/0x70
>   [<ffffffff81127410>] ? blkdev_get_by_dev+0x50/0x50
>   [<ffffffff812eef4f>] get_gendisk+0x2f/0x110
>   [<ffffffff81127410>] ? blkdev_get_by_dev+0x50/0x50
>   [<ffffffff81126e2c>] __blkdev_get+0x10c/0x3c0
>   [<ffffffff81127410>] ? blkdev_get_by_dev+0x50/0x50
>   [<ffffffff8112727d>] blkdev_get+0x19d/0x2e0
>   [<ffffffff81127410>] ? blkdev_get_by_dev+0x50/0x50
>   [<ffffffff81127466>] blkdev_open+0x56/0x70
>   [<ffffffff810f3e0f>] do_dentry_open.isra.19+0x1ff/0x310
>   [<ffffffff810f4aa3>] vfs_open+0x43/0x60
>   [<ffffffff81103009>] path_openat+0x2c9/0xf30
>   [<ffffffff81023c00>] ? __save_stack_trace+0x40/0xd0
>   [<ffffffff81104b79>] do_filp_open+0x79/0xd0
>   [<ffffffff81538219>] ? kmemleak_alloc+0x49/0xa0
>   [<ffffffff810f4e44>] do_sys_open+0x114/0x1e0
>   [<ffffffff810f4f29>] SyS_open+0x19/0x20
>   [<ffffffff8153c2e0>] entry_SYSCALL_64_fastpath+0x13/0x94
> 
> This patch adds the proper error check in hot_remove_store() not to
> call idr_remove() unconditionally.
> 
> Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=1010970
> Reported-and-tested-by: David Disseldorp <ddiss@xxxxxxx>
> Reviewed-by: David Disseldorp <ddiss@xxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>

Forgot to add Fixes tag:

Fixes: 17ec4cd98578 ("zram: don't call idr_remove() from zram_remove()")


Takashi


> ---
>  drivers/block/zram/zram_drv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 04365b17ee67..5163c8f918cb 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1403,7 +1403,8 @@ static ssize_t hot_remove_store(struct class *class,
>  	zram = idr_find(&zram_index_idr, dev_id);
>  	if (zram) {
>  		ret = zram_remove(zram);
> -		idr_remove(&zram_index_idr, dev_id);
> +		if (!ret)
> +			idr_remove(&zram_index_idr, dev_id);
>  	} else {
>  		ret = -ENODEV;
>  	}
> -- 
> 2.10.2
>