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Date: Fri, 3 Jan 2020 08:57:27 -0800 From: Eric Biggers <ebiggers@...nel.org> To: linux-fscrypt@...r.kernel.org Cc: keyrings@...r.kernel.org, linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net, linux-mtd@...ts.infradead.org, "Theodore Y . Ts'o" <tytso@....edu>, Jaegeuk Kim <jaegeuk@...nel.org>, David Howells <dhowells@...hat.com>, Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, Paul Crowley <paulcrowley@...gle.com>, Paul Lawrence <paullawrence@...gle.com>, Ondrej Mosnacek <omosnace@...hat.com>, Ondrej Kozina <okozina@...hat.com> Subject: Re: [PATCH v2] fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY On Tue, Nov 19, 2019 at 02:24:47PM -0800, Eric Biggers wrote: > From: Eric Biggers <ebiggers@...gle.com> > > Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be > specified by a Linux keyring key, rather than specified directly. > > This is useful because fscrypt keys belong to a particular filesystem > instance, so they are destroyed when that filesystem is unmounted. > Usually this is desired. But in some cases, userspace may need to > unmount and re-mount the filesystem while keeping the keys, e.g. during > a system update. This requires keeping the keys somewhere else too. > > The keys could be kept in memory in a userspace daemon. But depending > on the security architecture and assumptions, it can be preferable to > keep them only in kernel memory, where they are unreadable by userspace. > > We also can't solve this by going back to the original fscrypt API > (where for each file, the master key was looked up in the process's > keyring hierarchy) because that caused lots of problems of its own. > > Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a > Linux keyring key. This solves the problem by allowing userspace to (if > needed) save the keys securely in a Linux keyring for re-provisioning, > while still using the new fscrypt key management ioctls. > > This is analogous to how dm-crypt accepts a Linux keyring key, but the > key is then stored internally in the dm-crypt data structures rather > than being looked up again each time the dm-crypt device is accessed. > > Use a custom key type "fscrypt-provisioning" rather than one of the > existing key types such as "logon". This is strongly desired because it > enforces that these keys are only usable for a particular purpose: for > fscrypt as input to a particular KDF. Otherwise, the keys could also be > passed to any kernel API that accepts a "logon" key with any service > prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would > risk leaking information about the raw key despite it ostensibly being > unreadable. Of course, this mistake has already been made for multiple > kernel APIs; but since this is a new API, let's do it right. > > This patch has been tested using an xfstest which I wrote to test it. > > Signed-off-by: Eric Biggers <ebiggers@...gle.com> Applied to fscrypt.git#master for 5.6. - Eric
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