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From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-mtd@lists.infradead.org, linux-api@vger.kernel.org,
	linux-crypto@vger.kernel.org, keyrings@vger.kernel.org,
	Paul Crowley <paulcrowley@google.com>,
	Satya Tangirala <satyat@google.com>
Subject: Re: [PATCH v7 07/16] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl
Date: Mon, 12 Aug 2019 10:16:55 -0400
Message-ID: <20190812141655.GA11831@mit.edu> (raw)
In-Reply-To: <20190802043827.GA19201@sol.localdomain>

On Thu, Aug 01, 2019 at 09:38:27PM -0700, Eric Biggers wrote:
> 
> Here's a slightly updated version (I missed removing some stale text):

Apologies for the delaying in getting back.  Thanks, this looks great.

	      	  	      	      	     - Ted

> 
> Removing keys
> -------------
> 
> Two ioctls are available for removing a key that was added by
> `FS_IOC_ADD_ENCRYPTION_KEY`_:
> 
> - `FS_IOC_REMOVE_ENCRYPTION_KEY`_
> - `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_
> 
> These two ioctls differ only in cases where v2 policy keys are added
> or removed by non-root users.
> 
> These ioctls don't work on keys that were added via the legacy
> process-subscribed keyrings mechanism.
> 
> Before using these ioctls, read the `Kernel memory compromise`_
> section for a discussion of the security goals and limitations of
> these ioctls.
> 
> FS_IOC_REMOVE_ENCRYPTION_KEY
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> The FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master
> encryption key from the filesystem, and possibly removes the key
> itself.  It can be executed on any file or directory on the target
> filesystem, but using the filesystem's root directory is recommended.
> It takes in a pointer to a :c:type:`struct fscrypt_remove_key_arg`,
> defined as follows::
> 
>     struct fscrypt_remove_key_arg {
>             struct fscrypt_key_specifier key_spec;
>     #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY      0x00000001
>     #define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS     0x00000002
>             __u32 removal_status_flags;     /* output */
>             __u32 __reserved[5];
>     };
> 
> This structure must be zeroed, then initialized as follows:
> 
> - The key to remove is specified by ``key_spec``:
> 
>     - To remove a key used by v1 encryption policies, set
>       ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
>       in ``key_spec.u.descriptor``.  To remove this type of key, the
>       calling process must have the CAP_SYS_ADMIN capability in the
>       initial user namespace.
> 
>     - To remove a key used by v2 encryption policies, set
>       ``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
>       in ``key_spec.u.identifier``.
> 
> For v2 policy keys, this ioctl is usable by non-root users.  However,
> to make this possible, it actually just removes the current user's
> claim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY.
> Only after all claims are removed is the key really removed.
> 
> For example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000,
> then the key will be "claimed" by uid 1000, and
> FS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000.  Or, if
> both uids 1000 and 2000 added the key, then for each uid
> FS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim.  Only
> once *both* are removed is the key really removed.  (Think of it like
> unlinking a file that may have hard links.)
> 
> If FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also
> try to "lock" all files that had been unlocked with the key.  It won't
> lock files that are still in-use, so this ioctl is expected to be used
> in cooperation with userspace ensuring that none of the files are
> still open.  However, if necessary, the ioctl can be executed again
> later to retry locking any remaining files.
> 
> FS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed
> (but may still have files remaining to be locked), the user's claim to
> the key was removed, or the key was already removed but had files
> remaining to be the locked so the ioctl retried locking them.  In any
> of these cases, ``removal_status_flags`` is filled in with the
> following informational status flags:
> 
> - ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s)
>   are still in-use.  Not guaranteed to be set in the case where only
>   the user's claim to the key was removed.
> - ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the
>   user's claim to the key was removed, not the key itself
> 
> FS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors:
> 
> - ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type
>   was specified, but the caller does not have the CAP_SYS_ADMIN
>   capability in the initial user namespace
> - ``EINVAL``: invalid key specifier type, or reserved bits were set
> - ``ENOKEY``: the key object was not found at all, i.e. it was never
>   added in the first place or was already fully removed including all
>   files locked; or, the user does not have a claim to the key.
> - ``ENOTTY``: this type of filesystem does not implement encryption
> - ``EOPNOTSUPP``: the kernel was not configured with encryption
>   support for this filesystem, or the filesystem superblock has not
>   had encryption enabled on it
> 
> FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as
> `FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the
> ALL_USERS version of the ioctl will remove all users' claims to the
> key, not just the current user's.  I.e., the key itself will always be
> removed, no matter how many users have added it.  This difference is
> only meaningful if non-root users are adding and removing keys.
> 
> Because of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires
> "root", namely the CAP_SYS_ADMIN capability in the initial user
> namespace.  Otherwise it will fail with ``EACCES``.

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  reply index

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-26 22:41 [PATCH v7 00/16] fscrypt: key management improvements Eric Biggers
2019-07-26 22:41 ` [PATCH v7 01/16] fs, fscrypt: move uapi definitions to new header <linux/fscrypt.h> Eric Biggers
2019-07-28 15:08   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 02/16] fscrypt: use FSCRYPT_ prefix for uapi constants Eric Biggers
2019-07-26 22:41 ` [PATCH v7 03/16] fscrypt: use FSCRYPT_* definitions, not FS_* Eric Biggers
2019-07-26 22:41 ` [PATCH v7 04/16] fscrypt: add ->ci_inode to fscrypt_info Eric Biggers
2019-07-28 15:09   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 05/16] fscrypt: refactor v1 policy key setup into keysetup_legacy.c Eric Biggers
2019-07-28 15:40   ` Theodore Y. Ts'o
2019-07-29 19:37     ` Eric Biggers
2019-07-26 22:41 ` [PATCH v7 06/16] fscrypt: add FS_IOC_ADD_ENCRYPTION_KEY ioctl Eric Biggers
2019-07-28 18:50   ` Theodore Y. Ts'o
2019-07-29 19:46     ` Eric Biggers
2019-07-29 20:14       ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 07/16] fscrypt: add FS_IOC_REMOVE_ENCRYPTION_KEY ioctl Eric Biggers
2019-07-28 19:24   ` Theodore Y. Ts'o
2019-07-29 19:58     ` Eric Biggers
2019-07-31 18:38       ` Eric Biggers
2019-07-31 23:38         ` Theodore Y. Ts'o
2019-08-01  1:11           ` [f2fs-dev] " Eric Biggers
2019-08-01  5:31             ` Theodore Y. Ts'o
2019-08-01 18:35               ` Eric Biggers
2019-08-01 18:46                 ` Eric Biggers
2019-08-01 22:04               ` Eric Biggers
2019-08-02  4:38                 ` Eric Biggers
2019-08-12 14:16                   ` Theodore Y. Ts'o [this message]
2019-07-26 22:41 ` [PATCH v7 08/16] fscrypt: add FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl Eric Biggers
2019-07-28 19:30   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Eric Biggers
2019-07-28 19:39   ` Theodore Y. Ts'o
2019-07-29 20:29     ` Eric Biggers
2019-07-29 21:42       ` James Bottomley
2019-07-26 22:41 ` [PATCH v7 10/16] fscrypt: v2 encryption policy support Eric Biggers
2019-07-28 21:17   ` Theodore Y. Ts'o
2019-07-29 20:46     ` Eric Biggers
2019-07-26 22:41 ` [PATCH v7 11/16] fscrypt: allow unprivileged users to add/remove keys for v2 policies Eric Biggers
2019-07-28 21:22   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 12/16] fscrypt: require that key be added when setting a v2 encryption policy Eric Biggers
2019-07-28 21:24   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 13/16] ext4: wire up new fscrypt ioctls Eric Biggers
2019-07-28 21:24   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 14/16] f2fs: " Eric Biggers
2019-07-30  0:36   ` Jaegeuk Kim
2019-08-02  8:10   ` Chao Yu
2019-08-02 17:31     ` Eric Biggers
2019-08-04  9:42       ` [f2fs-dev] " Chao Yu
2019-07-26 22:41 ` [PATCH v7 15/16] ubifs: " Eric Biggers
2019-07-30  0:39   ` Theodore Y. Ts'o
2019-07-26 22:41 ` [PATCH v7 16/16] fscrypt: document the new ioctls and policy version Eric Biggers
2019-07-29  2:00   ` Theodore Y. Ts'o
2019-07-29 21:36     ` Eric Biggers

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