Re: [PATCH] ext4: don't allow journal inode to have encrypt flag

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Nov 01, 2022 at 10:33:12PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Mounting a filesystem whose journal inode has the encrypt flag causes a
> NULL dereference in fscrypt_limit_io_blocks() when the 'inlinecrypt'
> mount option is used.
> 
> The problem is that when jbd2_journal_init_inode() calls bmap(), it
> eventually finds its way into ext4_iomap_begin(), which calls
> fscrypt_limit_io_blocks().  fscrypt_limit_io_blocks() requires that if
> the inode is encrypted, then its encryption key must already be set up.
> That's not the case here, since the journal inode is never "opened" like
> a normal file would be.  Hence the crash.
> 
> A reproducer is:
> 
>     mkfs.ext4 -F /dev/vdb
>     debugfs -w /dev/vdb -R "set_inode_field <8> flags 0x80808"
>     mount /dev/vdb /mnt -o inlinecrypt
> 
> To fix this, make ext4 consider journal inodes with the encrypt flag to
> be invalid.  (Note, maybe other flags should be rejected on the journal
> inode too.  For now, this is just the minimal fix for the above issue.)
> 
> I've marked this as fixing the commit that introduced the call to
> fscrypt_limit_io_blocks(), since that's what made an actual crash start
> being possible.  But this fix could be applied to any version of ext4
> that supports the encrypt feature.
> 
> Reported-by: syzbot+ba9dac45bc76c490b7c3@syzkaller.appspotmail.com
> Fixes: 38ea50daa7a4 ("ext4: support direct I/O with fscrypt using blk-crypto")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/ext4/super.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 7950904fbf04f..2274f730b87e5 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -5723,7 +5723,7 @@ static struct inode *ext4_get_journal_inode(struct super_block *sb,
>  
>  	ext4_debug("Journal inode found at %p: %lld bytes\n",
>  		  journal_inode, journal_inode->i_size);
> -	if (!S_ISREG(journal_inode->i_mode)) {
> +	if (!S_ISREG(journal_inode->i_mode) || IS_ENCRYPTED(journal_inode)) {
>  		ext4_msg(sb, KERN_ERR, "invalid journal inode");
>  		iput(journal_inode);
>  		return NULL;
> 
> base-commit: 8f71a2b3f435f29b787537d1abedaa7d8ebe6647
> -- 

Ping.

- Eric