ext4: fix potential null deref in ext4_mb_init()

commit 3c3fac6 upstream.

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 Rust-for-Linux#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
Cc: stable@kernel.org
Fixes: f7eaacb ("ext4: convert free groups order lists to xarrays")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: LiBaokun96

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)