Re: [PATCH v5] ext4: fix potential use after free in system zone via remount with noblock_validity

zhangyi (F)
2019-08-27T09:20:50Z

On 2019/8/26 23:03, Jan Kara Wrote:
> On Mon 26-08-19 16:31:41, zhangyi (F) wrote:
>> On 2019/8/26 10:56, Theodore Y. Ts'o Wrote:
>>> I added a missing rcu_read_lock() to prevent a suspicious RCU
>>> warning when CONFIG_PROVE_RCU is enabled:
>>>
>>> diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
>>> index 003dc1dc2da3..f7bc914a74df 100644
>>> --- a/fs/ext4/block_validity.c
>>> +++ b/fs/ext4/block_validity.c
>>> @@ -330,11 +330,13 @@ void ext4_release_system_zone(struct super_block *sb)
>>> {
>>> struct ext4_system_blocks *system_blks;
>>>
>>> + rcu_read_lock();
>>> system_blks = rcu_dereference(EXT4_SB(sb)->system_blks);
>>> rcu_assign_pointer(EXT4_SB(sb)->system_blks, NULL);
>>>
>>> if (system_blks)
>>> call_rcu(&system_blks->rcu, ext4_destroy_system_zone);
>>> + rcu_read_unlock();
>>> }
>>>
>>> int ext4_data_block_valid(struct ext4_sb_info *sbi, ext4_fsblk_t start_blk,
>>>
>>
>> Hi Ted，
>> Sorry about missing this warning, I think switch to use:
>> system_blks = rcu_dereference_raw(EXT4_SB(sb)->system_blks);
>> or
>> system_blks = rcu_dereference_protected(EXT4_SB(sb)->system_blks, true);
>> is enough to fix this waring, am I missing something?
>
> Proper fix for this is actually using:
>
> system_blks = rcu_dereference_protected(EXT4_SB(sb)->system_blks,
> lockdep_is_held(&sb->s_umount));
>
Totally agree, will resend the patch.

Thanks,
Yi.