[PATCH 6.12 261/265] ext4: fix potential null deref in ext4_mb_init()

Greg Kroah-Hartman
2026-03-12T20:29:09Z

From: Baokun Li <libaokun1@huawei.com>

commit 3c3fac6bc0a9c00dbe65d8dc0d3a282afe4d3188 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 #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: f7eaacbb4e54 ("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>
---
fs/ext4/mballoc.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -3678,16 +3678,26 @@ static void ext4_discard_work(struct wor

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)