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

Jan Kara
2024-05-03T10:23:35Z

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

--
Jan Kara <jack@suse.com>
SUSE Labs, CR