Re: [PATCH v2 2/2] ext4: fix bug_on in __es_tree_search caused by wrong boot loader inode

[Jan Kara <jack@suse.cz>]

On Tue 25-10-22 10:26:14, Baokun Li wrote:
> On 2022/10/24 22:25, Jan Kara wrote:
> > On Fri 21-10-22 12:07:31, Baokun Li wrote:
> > > We got a issue as fllows:
> > > ...
> > > 
> > > In the above issue, ioctl invokes the swap_inode_boot_loader function to
> > > swap inode<5> and inode<12>. However, inode<5> contain incorrect imode and
> > > disordered extents, and i_nlink is set to 1. The extents check for inode in
> > > the ext4_iget function can be bypassed bacause 5 is EXT4_BOOT_LOADER_INO.
> > > While links_count is set to 1, the extents are not initialized in
> > > swap_inode_boot_loader. After the ioctl command is executed successfully,
> > > the extents are swapped to inode<12>, in this case, run the `cat` command
> > > to view inode<12>. And Bug_ON is triggered due to the incorrect extents.
> > > 
> > > When the boot loader inode is not initialized, its imode can be one of the
> > > following:
> > > 1) the imode is a bad type, which is marked as bad_inode in ext4_iget and
> > >     set to S_IFREG.
> > > 2) the imode is good type but not S_IFREG.
> > > 3) the imode is S_IFREG.
> > > 
> > > The BUG_ON may be triggered by bypassing the check in cases 1 and 2.
> > > Therefore, when the boot loader inode is bad_inode or its imode is not
> > > S_IFREG, initialize the inode to avoid triggering the BUG.
> > > 
> > > Signed-off-by: Baokun Li <libaokun1@huawei.com>
> > Grepping for calls to ext4_iget() in the ext4 code shows there are many
> > more places that will get unhappy (and crash) when ext4_iget() returns a
> > bad inode. In fact, I didn't find a place when returning bad inode would be
> > useful for anything. So why don't we just return EFSCORRUPTED instead of
> > returning a bad inode?
> > 
> > 								Honza
> 
> Hello Honza,
> 
> In ext4_iget(), the inode is marked as bad and returned only when ino is
> equal to
> EXT4_BOOT_LOADER_INO. In the error branch bad_inode, although the inode is
> marked as bad, the returned value is the corresponding error number.
> The boot loader inode is not initialized during mkfs. Therefore, when
> ext4_iget() is
> entered for the first time, imode of the inode is bad type. However, the
> swap_inode_boot_loader() needs to obtain the inode for initialization and
> swap.
> Therefore, a bad_inode is returned in ext4_iget.
> 
> Generally, ext4_iget() does not get the boot loader inode. Therefore, we
> only need
> to pay attention to the special inodes that can be specified.
> The following figure shows the check result:
> 
> 1) usr_quota_inum/grp_quota_inum/prj_quota_inum
> These inodes may be faulty. In the first patch, this situation is
> intercepted.
> At the beginning, FUZZ found that the quota inode was faulty. Later, we
> found that
> the operation function swap_inode_boot_loader() related to inode 5 was also
> faulty.
> 
> 2) journal_inum
> In ext4_get_journal_inode(), the system checks whether the imode is S_IFREG.
> Then,
> the bmap in jbd2_journal_init_inode() checks whether the inode has
> a_ops->bmap
> operation. The bad inode does not set the bmap operation, so there is no
> problem.
> 
> 3) last_orphan
> In ext4_orphan_get(), it checks if the imode is normal and if the inode is
> bad inode,
> so there is no problem.
> 
> 4) snapshot_inum
> No place to use snapshot_inum was found in the kernel, so there is no kernel
> issue.

Thanks for detailed explanation! Now I agree you have actually covered all
the cases. But since EXT4_BOOT_LOADER_INO has this special behavior maybe
it would be more robust to create a special iget flag for it? Like
EXT4_IGET_BAD? And only with this flag we'd be returning bad inode from
ext4_iget(), otherwise we always return the error code?

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