quota: Fix potential NULL pointer dereference

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: quota: Fix potential NULL pointer dereference Below race may cause NULL pointer dereference P1 P2 dquot_free_inode quota_off drop_dquot_ref remove_dquot_ref dquots = i_dquot(inode) dquots = i_dquot(inode) srcu_read_lock dquots[cnt]) != NULL (1) dquots[type] = NULL (2) spin_lock(&dquots[cnt]->dq_dqb_lock) (3) .... If dquot_free_inode(or other routines) checks inode's quota pointers (1) before quota_off sets it to NULL(2) and use it (3) after that, NULL pointer dereference will be triggered. So let's fix it by using a temporary pointer to avoid this issue.

References

• https://git.kernel.org/stable/c/8514899c1a4edf802f03c408db901063aa3f05a1
• https://git.kernel.org/stable/c/49669f8e7eb053f91d239df7b1bfb4500255a9d0
• https://git.kernel.org/stable/c/61380537aa6dd32d8a723d98b8f1bd1b11d8fee0
• https://git.kernel.org/stable/c/1ca72a3de915f87232c9a4cb9bebbd3af8ed3e25
• https://git.kernel.org/stable/c/7f9e833fc0f9b47be503af012eb5903086939754
• https://git.kernel.org/stable/c/40a673b4b07efd6f74ff3ab60f38b26aa91ee5d5
• https://git.kernel.org/stable/c/f2649d98aa9ca8623149b3cb8df00c944f5655c7
• https://git.kernel.org/stable/c/6afc9f4434fa8063aa768c2bf5bf98583aee0877
• https://git.kernel.org/stable/c/d0aa72604fbd80c8aabb46eda00535ed35570f1f