Re: [PATCH v2 3/5] ext4: add sanity check for encryption + DAX

[Ross Zwisler]

On Tue, Sep 12, 2017 at 08:45:00AM +0200, Jan Kara wrote:
> On Mon 11-09-17 23:05:24, Ross Zwisler wrote:
> > We prevent DAX from being used on inodes which are using ext4's built in
> > encryption via a check in ext4_set_inode_flags().  We do have what appears
> > to be an unsafe transition of S_DAX in ext4_set_context(), though, where
> > S_DAX can get disabled without us doing a proper writeback + invalidate.
> > 
> > There are also issues with mm-level races when changing the value of S_DAX,
> > as well as issues with the VM_MIXEDMAP flag:
> > 
> > https://www.spinics.net/lists/linux-xfs/msg09859.html
> > 
> > I actually think we are safe in this case because of the following:
> > 
> > 1) You can't encrypt an existing file.  Encryption can only be set on an
> > empty directory, with new inodes in that directory being created with
> > encryption turned on, so I don't think it's possible to turn encryption on
> > for a file that has open DAX mmaps or outstanding I/Os.
> > 
> > 2) There is no way to turn encryption off on a given file.  Once an inode
> > is encrypted, it stays encrypted for the life of that inode, so we don't
> > have to worry about the case where we turn encryption off and S_DAX
> > suddenly turns on.
> > 
> > 3) The only way we end up in ext4_set_context() to turn on encryption is
> > when we are creating a new file in the encrypted directory.  This happens
> > as part of ext4_create() before the inode has been allowed to do any I/O.
> > Here's the call tree:
> > 
> >  ext4_create()
> >    __ext4_new_inode()
> > 	 ext4_set_inode_flags() // sets S_DAX
> > 	 fscrypt_inherit_context()
> > 		fscrypt_get_encryption_info();
> > 		ext4_set_context() // sets EXT4_INODE_ENCRYPT, clears S_DAX
> > 
> > So, I actually think it's safe to transition S_DAX in ext4_set_context()
> > without any locking, writebacks or invalidations.  I've added a
> > WARN_ON_ONCE() sanity check to make sure that we are notified if we ever
> > encounter a case where we are encrypting an inode that already has data,
> > in which case we need to add code to safely transition S_DAX.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> > CC: stable@xxxxxxxxxxxxxxx
> 
> Looks good to me - and frankly I think we can drop the stable CC here...

Sure, I'm fine to drop the CC to stable.

> Anyway, you can add:
> 
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> 
> 								Honza
> 
> > ---
> >  fs/ext4/super.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> > index 4251e50..c090780 100644
> > --- a/fs/ext4/super.c
> > +++ b/fs/ext4/super.c
> > @@ -1159,6 +1159,9 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
> >  	if (inode->i_ino == EXT4_ROOT_INO)
> >  		return -EPERM;
> >  
> > +	if (WARN_ON_ONCE(IS_DAX(inode) && i_size_read(inode)))
> > +		return -EINVAL;
> > +
> >  	res = ext4_convert_inline_data(inode);
> >  	if (res)
> >  		return res;
> > -- 
> > 2.9.5
> > 
> -- 
> Jan Kara <jack@xxxxxxxx>
> SUSE Labs, CR