linux-kernel - Re: [PATCH v3 fs/btrfs v3] btrfs: fix memory leak of qgroup_list in btrfs_add_qgroup

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Message-ID: <bdfd4ab9-3db1-4ce3-8f9a-8e4fceb5488a@suse.com>
Date: Mon, 27 Oct 2025 16:12:02 +1030
From: Qu Wenruo <wqu@...e.com>
To: Shardul Bankar <shardulsb08@...il.com>, linux-btrfs@...r.kernel.org
Cc: clm@...com, dsterba@...e.com, linux-kernel@...r.kernel.org,
 fdmanana@...nel.org
Subject: Re: [PATCH v3 fs/btrfs v3] btrfs: fix memory leak of qgroup_list in
 btrfs_add_qgroup_relation



在 2025/10/26 06:30, Shardul Bankar 写道:

You don't need to include the "fs/btrfs v3" in the [PATCH] part.

You can refer to the mailing list for how to add versions to the patches.
Thankfully the "fs/btrfs v3" part will just be discarded by git-am.

> When btrfs_add_qgroup_relation() is called with invalid qgroup levels
> (src >= dst), the function returns -EINVAL directly without freeing the
> preallocated qgroup_list structure passed by the caller. This causes a
> memory leak because the caller unconditionally sets the pointer to NULL
> after the call, preventing any cleanup.
> 
> The issue occurs because the level validation check happens before the
> mutex is acquired and before any error handling path that would free
> the prealloc pointer. On this early return, the cleanup code at the
> 'out' label (which includes kfree(prealloc)) is never reached.
> 
> In btrfs_ioctl_qgroup_assign(), the code pattern is:
> 
>      prealloc = kzalloc(sizeof(*prealloc), GFP_KERNEL);
>      ret = btrfs_add_qgroup_relation(trans, sa->src, sa->dst, prealloc);
>      prealloc = NULL;  // Always set to NULL regardless of return value
>      ...
>      kfree(prealloc);  // This becomes kfree(NULL), does nothing
> 
> When the level check fails, 'prealloc' is never freed by either the
> callee or the caller, resulting in a 64-byte memory leak per failed
> operation. This can be triggered repeatedly by an unprivileged user
> with access to a writable btrfs mount, potentially exhausting kernel
> memory.
> 
> Fix this by freeing prealloc before the early return, ensuring prealloc
> is always freed on all error paths.
> 
> Fixes: 4addc1ffd67a ("btrfs: qgroup: preallocate memory before adding a relation")
> Signed-off-by: Shardul Bankar <shardulsb08@...il.com>

Reviewed-by: Qu Wenruo <wqu@...e.com>

And now pushed to for-next branch.

Thanks,
Qu

> ---
> 
> v3:
>   - Update Fixes: tag to correct commit SHA as suggested by Filipe Manana.
>   - No code changes.
> 
> v2:
>   - Free prealloc directly before returning -EINVAL (no mutex held),
>     per review from Qu Wenruo.
>   - Drop goto-based cleanup.
> 
>   fs/btrfs/qgroup.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 1175b8192cd7..31ad8580322a 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -1539,8 +1539,10 @@ int btrfs_add_qgroup_relation(struct btrfs_trans_handle *trans, u64 src, u64 dst
>   	ASSERT(prealloc);
>   
>   	/* Check the level of src and dst first */
> -	if (btrfs_qgroup_level(src) >= btrfs_qgroup_level(dst))
> +	if (btrfs_qgroup_level(src) >= btrfs_qgroup_level(dst)) {
> +		kfree(prealloc);
>   		return -EINVAL;
> +	}
>   
>   	mutex_lock(&fs_info->qgroup_ioctl_lock);
>   	if (!fs_info->quota_root) {