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