From: Eric Biggers Subject: Re: [PATCH] fscrypt: remove broken support for detecting keyring key revocation Date: Tue, 14 Mar 2017 16:48:42 -0700 Message-ID: <20170314234842.GB127087@gmail.com> References: <20170221230711.85222-1-ebiggers3@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fscrypt@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org, "Theodore Y . Ts'o" , Jaegeuk Kim , Richard Weinberger , Anand Jain , Eric Biggers , stable@vger.kernel.org To: linux-fsdevel@vger.kernel.org Return-path: Content-Disposition: inline In-Reply-To: <20170221230711.85222-1-ebiggers3@gmail.com> Sender: stable-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Tue, Feb 21, 2017 at 03:07:11PM -0800, Eric Biggers wrote: > From: Eric Biggers > > Filesystem encryption ostensibly supported revoking a keyring key that > had been used to "unlock" encrypted files, causing those files to become > "locked" again. This was, however, buggy for several reasons, the most > severe of which was that when key revocation happened to be detected for > an inode, its fscrypt_info was immediately freed, even while other > threads could be using it for encryption or decryption concurrently. > This could be exploited to crash the kernel or worse. > > This patch fixes the use-after-free by removing the code which detects > the keyring key having been revoked, invalidated, or expired. Instead, > an encrypted inode that is "unlocked" now simply remains unlocked until > it is evicted from memory. Note that this is no worse than the case for > block device-level encryption, e.g. dm-crypt, and it still remains > possible for a privileged user to evict unused pages, inodes, and > dentries by running 'sync; echo 3 > /proc/sys/vm/drop_caches', or by > simply unmounting the filesystem. In fact, one of those actions was > already needed anyway for key revocation to work even somewhat sanely. > This change is not expected to break any applications. > > In the future I'd like to implement a real API for fscrypt key > revocation that interacts sanely with ongoing filesystem operations --- > waiting for existing operations to complete and blocking new operations, > and invalidating and sanitizing key material and plaintext from the VFS > caches. But this is a hard problem, and for now this bug must be fixed. > > This bug affected almost all versions of ext4, f2fs, and ubifs > encryption, and it was potentially reachable in any kernel configured > with encryption support (CONFIG_EXT4_ENCRYPTION=y, > CONFIG_EXT4_FS_ENCRYPTION=y, CONFIG_F2FS_FS_ENCRYPTION=y, or > CONFIG_UBIFS_FS_ENCRYPTION=y). Note that older kernels did not use the > shared fs/crypto/ code, but due to the potential security implications > of this bug, it may still be worthwhile to backport this fix to them. > > Fixes: b7236e21d55f ("ext4 crypto: reorganize how we store keys in the inode") > Cc: stable@vger.kernel.org # v4.2+ > Signed-off-by: Eric Biggers Ted, can this be sent to Linus soon? This needs to be fixed as it's a security vulnerability on some systems. Eric