Re: [PATCH] ext4: fix potential null deref in ext4_mb_init()

Zhang Yi
2025-08-25T04:39:55Z

On 8/25/2025 11:38 AM, libaokun@huaweicloud.com wrote:
> From: Baokun Li <libaokun1@huawei.com>
>
> In ext4_mb_init(), ext4_mb_avg_fragment_size_destroy() may be called
> when sbi->s_mb_avg_fragment_size remains uninitialized (e.g., if groupinfo
> slab cache allocation fails). Since ext4_mb_avg_fragment_size_destroy()
> lacks null pointer checking, this leads to a null pointer dereference.
>
> ==================================================================
> EXT4-fs: no memory for groupinfo slab cache
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> PGD 0 P4D 0
> Oops: Oops: 0002 [#1] SMP PTI
> CPU:2 UID: 0 PID: 87 Comm:mount Not tainted 6.17.0-rc2 #1134 PREEMPT(none)
> RIP: 0010:_raw_spin_lock_irqsave+0x1b/0x40
> Call Trace:
> <TASK>
> xa_destroy+0x61/0x130
> ext4_mb_init+0x483/0x540
> __ext4_fill_super+0x116d/0x17b0
> ext4_fill_super+0xd3/0x280
> get_tree_bdev_flags+0x132/0x1d0
> vfs_get_tree+0x29/0xd0
> do_new_mount+0x197/0x300
> __x64_sys_mount+0x116/0x150
> do_syscall_64+0x50/0x1c0
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> ==================================================================
>
> Therefore, add necessary null check to ext4_mb_avg_fragment_size_destroy()
> to prevent this issue. The same fix is also applied to
> ext4_mb_largest_free_orders_destroy().
>
> Reported-by: syzbot+1713b1aa266195b916c2@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=1713b1aa266195b916c2
> Fixes: f7eaacbb4e54 ("ext4: convert free groups order lists to xarrays")
> Signed-off-by: Baokun Li <libaokun1@huawei.com>

Looks good to me.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

> ---
> fs/ext4/mballoc.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 5898d92ba19f..6070d3c86678 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3655,16 +3655,26 @@ static void ext4_discard_work(struct work_struct *work)
>
> static inline void ext4_mb_avg_fragment_size_destroy(struct ext4_sb_info *sbi)
> {
> + if (!sbi->s_mb_avg_fragment_size)
> + return;
> +
> for (int i = 0; i < MB_NUM_ORDERS(sbi->s_sb); i++)
> xa_destroy(&sbi->s_mb_avg_fragment_size[i]);
> +
> kfree(sbi->s_mb_avg_fragment_size);
> + sbi->s_mb_avg_fragment_size = NULL;
> }
>
> static inline void ext4_mb_largest_free_orders_destroy(struct ext4_sb_info *sbi)
> {
> + if (!sbi->s_mb_largest_free_orders)
> + return;
> +
> for (int i = 0; i < MB_NUM_ORDERS(sbi->s_sb); i++)
> xa_destroy(&sbi->s_mb_largest_free_orders[i]);
> +
> kfree(sbi->s_mb_largest_free_orders);
> + sbi->s_mb_largest_free_orders = NULL;
> }
>
> int ext4_mb_init(struct super_block *sb)