Re: [PATCH] fscrypt: don't evict dirty inodes after removing key

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Mar 05, 2020 at 12:41:38AM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> After FS_IOC_REMOVE_ENCRYPTION_KEY removes a key, it syncs the
> filesystem and tries to get and put all inodes that were unlocked by the
> key so that unused inodes get evicted via fscrypt_drop_inode().
> Normally, the inodes are all clean due to the sync.
> 
> However, after the filesystem is sync'ed, userspace can modify and close
> one of the files.  (Userspace is *supposed* to close the files before
> removing the key.  But it doesn't always happen, and the kernel can't
> assume it.)  This causes the inode to be dirtied and have i_count == 0.
> Then, fscrypt_drop_inode() failed to consider this case and indicated
> that the inode can be dropped, causing the write to be lost.
> 
> On f2fs, other problems such as a filesystem freeze could occur due to
> the inode being freed while still on f2fs's dirty inode list.
> 
> Fix this bug by making fscrypt_drop_inode() only drop clean inodes.
> 
> I've written an xfstest which detects this bug on ext4, f2fs, and ubifs.
> 
> Fixes: b1c0ec3599f4 ("fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl")
> Cc: <stable@vger.kernel.org> # v5.4+
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/crypto/keysetup.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
> index 65cb09fa6ead..08c9f216a54d 100644
> --- a/fs/crypto/keysetup.c
> +++ b/fs/crypto/keysetup.c
> @@ -538,6 +538,15 @@ int fscrypt_drop_inode(struct inode *inode)
>  		return 0;
>  	mk = ci->ci_master_key->payload.data[0];
>  
> +	/*
> +	 * With proper, non-racy use of FS_IOC_REMOVE_ENCRYPTION_KEY, all inodes
> +	 * protected by the key were cleaned by sync_filesystem().  But if
> +	 * userspace is still using the files, inodes can be dirtied between
> +	 * then and now.  We mustn't lose any writes, so skip dirty inodes here.
> +	 */
> +	if (inode->i_state & I_DIRTY_ALL)
> +		return 0;
> +
>  	/*
>  	 * Note: since we aren't holding ->mk_secret_sem, the result here can
>  	 * immediately become outdated.  But there's no correctness problem with
> -- 

Applied to fscrypt.git#for-stable for 5.6.

- Eric