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

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Message-Id: <20251025092951.2866847-1-shardulsb08@gmail.com>
Date: Sat, 25 Oct 2025 14:59:51 +0530
From: Shardul Bankar <shardulsb08@...il.com>
To: linux-btrfs@...r.kernel.org
Cc: clm@...com,
	dsterba@...e.com,
	linux-kernel@...r.kernel.org,
	shardulsb08@...il.com
Subject: [PATCH fs/btrfs] btrfs: fix memory leak of qgroup_list in btrfs_add_qgroup_relation

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 changing the early return to a goto that reaches the
cleanup code, ensuring prealloc is always freed on all error paths.

Reported-by: BRF (btrfs runtime fuzzer)
Fixes: 8465ecec9611 ("btrfs: Check qgroup level in kernel qgroup assign.")
Signed-off-by: Shardul Bankar <shardulsb08@...il.com>
---
 fs/btrfs/qgroup.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 1175b8192cd7..0a25bfdd442f 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))
-		return -EINVAL;
+	if (btrfs_qgroup_level(src) >= btrfs_qgroup_level(dst)) {
+		ret = -EINVAL;
+		goto out;
+	}

 	mutex_lock(&fs_info->qgroup_ioctl_lock);
 	if (!fs_info->quota_root) {
-- 
2.34.1