Re: [PATCH] btrfs: qgroup: fix sleep from invalid context bug in update_qgroup_limit_item()

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/11/10 22:13, ChenXiaoSong wrote:
> Syzkaller reported BUG as follows:
> 
>    BUG: sleeping function called from invalid context at
>         include/linux/sched/mm.h:274
>    Call Trace:
>     <TASK>
>     dump_stack_lvl+0xcd/0x134
>     __might_resched.cold+0x222/0x26b
>     kmem_cache_alloc+0x2e7/0x3c0
>     update_qgroup_limit_item+0xe1/0x390
>     btrfs_qgroup_inherit+0x147b/0x1ee0
>     create_subvol+0x4eb/0x1710
>     btrfs_mksubvol+0xfe5/0x13f0
>     __btrfs_ioctl_snap_create+0x2b0/0x430
>     btrfs_ioctl_snap_create_v2+0x25a/0x520
>     btrfs_ioctl+0x2a1c/0x5ce0
>     __x64_sys_ioctl+0x193/0x200
>     do_syscall_64+0x35/0x80
> 
> Fix this by introducing __update_qgroup_limit_item() helper, allocate
> memory outside of the spin lock.
> 
> Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>

Unfortunately, __update_qgroup_limit_item() can still sleep.

As it calls btrfs_search_slot(), which can lead to disk IO if the qgroup 
tree is not cached.


I believe the proper way is to either unlock the spinlock inside 
btrfs_qgroup_inherit() (which needs extra scrutiny on the qgroup lock), 
or delayed the limit item updates until we have unlocked the spinlock.

To me, the latter one seems more reasonable, as it's just one qgroup 
(@dstgroup), and we're doing the same delayed work for sysfs interface 
creation.

Thanks,
Qu

> ---
>   fs/btrfs/qgroup.c | 35 ++++++++++++++++++++++++++---------
>   1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 9334c3157c22..99a61cc04b68 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -768,11 +768,11 @@ static int del_qgroup_item(struct btrfs_trans_handle *trans, u64 qgroupid)
>   	return ret;
>   }
>   
> -static int update_qgroup_limit_item(struct btrfs_trans_handle *trans,
> -				    struct btrfs_qgroup *qgroup)
> +static int __update_qgroup_limit_item(struct btrfs_trans_handle *trans,
> +				      struct btrfs_qgroup *qgroup,
> +				      struct btrfs_path *path) >   {
>   	struct btrfs_root *quota_root = trans->fs_info->quota_root;
> -	struct btrfs_path *path;
>   	struct btrfs_key key;
>   	struct extent_buffer *l;
>   	struct btrfs_qgroup_limit_item *qgroup_limit;
> @@ -783,10 +783,6 @@ static int update_qgroup_limit_item(struct btrfs_trans_handle *trans,
>   	key.type = BTRFS_QGROUP_LIMIT_KEY;
>   	key.offset = qgroup->qgroupid;
>   
> -	path = btrfs_alloc_path();
> -	if (!path)
> -		return -ENOMEM;
> -
>   	ret = btrfs_search_slot(trans, quota_root, &key, path, 0, 1);
>   	if (ret > 0)
>   		ret = -ENOENT;
> @@ -806,6 +802,21 @@ static int update_qgroup_limit_item(struct btrfs_trans_handle *trans,
>   	btrfs_mark_buffer_dirty(l);
>   
>   out:
> +	return ret;
> +}
> +
> +static int update_qgroup_limit_item(struct btrfs_trans_handle *trans,
> +				    struct btrfs_qgroup *qgroup)
> +{
> +	struct btrfs_path *path;
> +	int ret;
> +
> +	path = btrfs_alloc_path();
> +	if (!path)
> +		return -ENOMEM;
> +
> +	ret = __update_qgroup_limit_item(trans, qgroup, path);
> +
>   	btrfs_free_path(path);
>   	return ret;
>   }
> @@ -2860,6 +2871,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>   	bool need_rescan = false;
>   	u32 level_size = 0;
>   	u64 nums;
> +	struct btrfs_path *path;
>   
>   	/*
>   	 * There are only two callers of this function.
> @@ -2935,6 +2947,11 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>   		ret = 0;
>   	}
>   
> +	path = btrfs_alloc_path();
> +	if (!path) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
>   
>   	spin_lock(&fs_info->qgroup_lock);
>   
> @@ -2950,8 +2967,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>   		dstgroup->max_excl = inherit->lim.max_excl;
>   		dstgroup->rsv_rfer = inherit->lim.rsv_rfer;
>   		dstgroup->rsv_excl = inherit->lim.rsv_excl;
> -
> -		ret = update_qgroup_limit_item(trans, dstgroup);
> +		ret = __update_qgroup_limit_item(trans, dstgroup, path);
>   		if (ret) {
>   			qgroup_mark_inconsistent(fs_info);
>   			btrfs_info(fs_info,
> @@ -3053,6 +3069,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans, u64 srcid,
>   
>   unlock:
>   	spin_unlock(&fs_info->qgroup_lock);
> +	btrfs_free_path(path);
>   	if (!ret)
>   		ret = btrfs_sysfs_add_one_qgroup(fs_info, dstgroup);
>   out: