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... 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
