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SubjectRe: [PATCH] ext4: fix null-ptr-deref in ext4_write_info
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在 2022/8/5 21:07, Lukas Czerner 写道:
> On Fri, Aug 05, 2022 at 08:39:47PM +0800, Baokun Li wrote:
>> I caught a null-ptr-deref bug as follows:
>> ==================================================================
>> KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
>> CPU: 1 PID: 1589 Comm: umount Not tainted 5.10.0-02219-dirty #339
>> RIP: 0010:ext4_write_info+0x53/0x1b0
>> [...]
>> Call Trace:
>> dquot_writeback_dquots+0x341/0x9a0
>> ext4_sync_fs+0x19e/0x800
>> __sync_filesystem+0x83/0x100
>> sync_filesystem+0x89/0xf0
>> generic_shutdown_super+0x79/0x3e0
>> kill_block_super+0xa1/0x110
>> deactivate_locked_super+0xac/0x130
>> deactivate_super+0xb6/0xd0
>> cleanup_mnt+0x289/0x400
>> __cleanup_mnt+0x16/0x20
>> task_work_run+0x11c/0x1c0
>> exit_to_user_mode_prepare+0x203/0x210
>> syscall_exit_to_user_mode+0x5b/0x3a0
>> do_syscall_64+0x59/0x70
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> ==================================================================
>>
>> Above issue may happen as follows:
>> -------------------------------------
>> exit_to_user_mode_prepare
>> task_work_run
>> __cleanup_mnt
>> cleanup_mnt
>> deactivate_super
>> deactivate_locked_super
>> kill_block_super
>> generic_shutdown_super
>> shrink_dcache_for_umount
>> dentry = sb->s_root
>> sb->s_root = NULL <--- Here set NULL
>> sync_filesystem
>> __sync_filesystem
>> sb->s_op->sync_fs > ext4_sync_fs
>> dquot_writeback_dquots
>> sb->dq_op->write_info > ext4_write_info
>> ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2)
>> d_inode(sb->s_root)
>> s_root->d_inode <--- Null pointer dereference
>>
>> To solve this problem, we use ext4_journal_start_sb directly
>> to avoid s_root being used.
> Are we syncing the file system after the superblock shutdown and getting
> away with it? This does not look good. Do you have a reproducer?
>
> Thanks!
> -Lukas

Hi, Lukas!

This problem is triggered by a pressure test when I reproduce another
problem.

So I didn't have the reproducer.

I looked at the error stack code and found that it seems there is
something wrong.

Moreover, it's really weird to write code like
"ext4_journal_start(d_inode(sb->s_root), ...)".

>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
>> ---
>> fs/ext4/super.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index 9a66abcca1a8..0ce4565422f6 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -6653,7 +6653,7 @@ static int ext4_write_info(struct super_block *sb, int type)
>> handle_t *handle;
>>
>> /* Data block + inode block */
>> - handle = ext4_journal_start(d_inode(sb->s_root), EXT4_HT_QUOTA, 2);
>> + handle = ext4_journal_start_sb(sb, EXT4_HT_QUOTA, 2);
>> if (IS_ERR(handle))
>> return PTR_ERR(handle);
>> ret = dquot_commit_info(sb, type);
>> --
>> 2.31.1
>>
> .


Thanks!

--
With Best Regards,
Baokun Li

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