Re: [syzbot] [ext4?] WARNING in mb_cache_destroy

Baokun Li
2024-05-03T11:38:25Z

On 2024/5/3 18:23, Jan Kara wrote:
> Hi!
>
> On Fri 03-05-24 17:51:07, Baokun Li wrote:
>> On 2024/5/2 18:33, Jan Kara wrote:
>>> On Tue 30-04-24 08:04:03, syzbot wrote:
>>>> syzbot has bisected this issue to:
>>>>
>>>> commit 67d7d8ad99beccd9fe92d585b87f1760dc9018e3
>>>> Author: Baokun Li <libaokun1@huawei.com>
>>>> Date: Thu Jun 16 02:13:56 2022 +0000
>>>>
>>>> ext4: fix use-after-free in ext4_xattr_set_entry
>>> So I'm not sure the bisect is correct since the change is looking harmless.
>> Yes, the root cause of the problem has nothing to do with this patch,
>> and please see the detailed analysis below.
>>> But it is sufficiently related that there indeed may be some relationship.
>>> Anyway, the kernel log has:
>>>
>>> [ 44.932900][ T1063] EXT4-fs warning (device loop0): ext4_evict_inode:297: xattr delete (err -12)
>>> [ 44.943316][ T1063] EXT4-fs (loop0): unmounting filesystem.
>>> [ 44.949531][ T1063] ------------[ cut here ]------------
>>> [ 44.955050][ T1063] WARNING: CPU: 0 PID: 1063 at fs/mbcache.c:409 mb_cache_destroy+0xda/0x110
>>>
>>> So ext4_xattr_delete_inode() called when removing inode has failed with
>>> ENOMEM and later mb_cache_destroy() was eventually complaining about having
>>> mbcache entry with increased refcount. So likely some error cleanup path is
>>> forgetting to drop mbcache entry reference somewhere but at this point I
>>> cannot find where. We'll likely need to play with the reproducer to debug
>>> that. Baokun, any chance for looking into this?
>>>
>>> Honza
>> As you guessed, when -ENOMEM is returned in ext4_sb_bread(),
>> the reference count of ce is not properly released, as follows.
>>
>> ext4_create
>>  __ext4_new_inode
>>   security_inode_init_security
>>    ext4_initxattrs
>>     ext4_xattr_set_handle
>>      ext4_xattr_block_find
>>      ext4_xattr_block_set
>>       ext4_xattr_block_cache_find
>>         ce = mb_cache_entry_find_first
>>             __entry_find
>>             atomic_inc_not_zero(&entry->e_refcnt)
>>         bh = ext4_sb_bread(inode->i_sb, ce->e_value, REQ_PRIO);
>>         if (PTR_ERR(bh) == -ENOMEM)
>>             return NULL;
>>
>> Before merging into commit 67d7d8ad99be("ext4: fix use-after-free
>> in ext4_xattr_set_entry"), it will not return early in
>> ext4_xattr_ibody_find(),
>> so it tries to find it in iboy, fails the check in xattr_check_inode() and
>> returns without executing ext4_xattr_block_find(). Thus it will bisect
>> the patch, but actually has nothing to do with it.
>>
>> ext4_xattr_ibody_get
>>  xattr_check_inode
>>   __xattr_check_inode
>>    check_xattrs
>>     if (end - (void *)header < sizeof(*header) + sizeof(u32))
>>       "in-inode xattr block too small"
>>
>> Here's the patch in testing, I'll send it out officially after it is tested.
>> (PS:  I'm not sure if propagating the ext4_xattr_block_cache_find() errors
>> would be better.)
> Great! Thanks for debugging this! Some comments to your fix below:
>
>> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
>> index b67a176bfcf9..5c9e751915fd 100644
>> --- a/fs/ext4/xattr.c
>> +++ b/fs/ext4/xattr.c
>> @@ -3113,11 +3113,10 @@ ext4_xattr_block_cache_find(struct inode *inode,
>>
>>          bh = ext4_sb_bread(inode->i_sb, ce->e_value, REQ_PRIO);
>>          if (IS_ERR(bh)) {
>> -            if (PTR_ERR(bh) == -ENOMEM)
>> -                return NULL;
>> +            if (PTR_ERR(bh) != -ENOMEM)
>> +                EXT4_ERROR_INODE(inode, "block %lu read error",
>> +                         (unsigned long)ce->e_value);
>>              bh = NULL;
>> -            EXT4_ERROR_INODE(inode, "block %lu read error",
>> -                     (unsigned long)ce->e_value);
>>          } else if (ext4_xattr_cmp(header, BHDR(bh)) == 0) {
>>              *pce = ce;
>>              return bh;
> So if we get the ENOMEM error, continuing the iteration seems to be
> pointless as we'll likely get it for the following entries as well. I think
> the original behavior of aborting the iteration in case of ENOMEM is
> actually better. We just have to do mb_cache_entry_put(ea_block_cache, ce)
> before returning...
>
> Honza
Returning NULL here would normally attempt to allocate a new
xattr_block in ext4_xattr_block_set(), and when ext4_sb_bread() fails,
allocating the new block and inserting it would most likely fail as well,
so my initial thought was to propagate the error from ext4_sb_bread()
to also make ext4_xattr_block_set() fail when ext4_sb_bread() fails.

But I noticed that before Ted added the special handling for -ENOMEM,
EXT4_ERROR_INODE was called to set the ERROR_FS flag no matter
what error ext4_sb_bread() returned, and after we can distinguish
between -EIO and -ENOMEM, we don't have to set the ERROR_FS flag
in the case of -ENOMEM. So there's this conservative fix now.

In short, in my personal opinion, for -EIO and -ENOMEM, they should
be the same except whether or not the ERROR_FS flag is set.
Otherwise, I think adding mb_cache_entry_put() directly is the easiest
and most straightforward fix.  Honza, do you have any other thoughts?

Thanks,
Baokun