ACK/Cmnt: [PATCH] ext4: fail ext4_iget for root directory if unallocated

[Stefan Bader]

On 25.07.2018 13:43, Paolo Pisati wrote:
> From: Theodore Ts'o <tytso at mit.edu>
> 
> CVE-2018-1092
> 
> If the root directory has an i_links_count of zero, then when the file
> system is mounted, then when ext4_fill_super() notices the problem and
> tries to call iput() the root directory in the error return path,
> ext4_evict_inode() will try to free the inode on disk, before all of
> the file system structures are set up, and this will result in an OOPS
> caused by a NULL pointer dereference.
> 
> This issue has been assigned CVE-2018-1092.
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=199179
> https://bugzilla.redhat.com/show_bug.cgi?id=1560777
> 
> Reported-by: Wen Xu <wen.xu at gatech.edu>
> Signed-off-by: Theodore Ts'o <tytso at mit.edu>
> Cc: stable at vger.kernel.org
> (backported from commit 8e4b5eae5decd9dfe5a4ee369c22028f90ab4c44)
maybe...
[ppisati: replaced EFSCORRUPTED with EIO]
> Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>
> ---
>  fs/ext4/inode.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index f840c90..e213e00 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -4132,6 +4132,12 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
>  		goto bad_inode;
>  	raw_inode = ext4_raw_inode(&iloc);
>  
> +	if ((ino == EXT4_ROOT_INO) && (raw_inode->i_links_count == 0)) {
> +		EXT4_ERROR_INODE(inode, "root inode unallocated");
> +		ret = -EIO;
> +		goto bad_inode;
> +	}
> +
>  	if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE) {
>  		ei->i_extra_isize = le16_to_cpu(raw_inode->i_extra_isize);
>  		if (EXT4_GOOD_OLD_INODE_SIZE + ei->i_extra_isize >
> 


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