[RFC,v2] ima,fuse: introduce new fs flag FS_NO_IMA_CACHE

Message ID	20180116151000.443-1-alban@kinvolk.io (mailing list archive)
State	New, archived
Headers	show Return-Path: <linux-security-module-owner@kernel.org> From: Alban Crequy <alban.crequy@gmail.com> To: Alban Crequy <alban@kinvolk.io> Cc: iago@kinvolk.io, dongsu@kinvolk.io, linux-kernel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, linux-fsdevel@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>, Alexander Viro <viro@zeniv.linux.org.uk>, Mimi Zohar <zohar@linux.vnet.ibm.com>, Dmitry Kasatkin <dmitry.kasatkin@gmail.com>, James Morris <james.l.morris@oracle.com>, "Serge E. Hallyn" <serge@hallyn.com>, Seth Forshee <seth.forshee@canonical.com>, Christoph Hellwig <hch@infradead.org> Subject: [RFC PATCH v2] ima,fuse: introduce new fs flag FS_NO_IMA_CACHE Date: Tue, 16 Jan 2018 16:10:00 +0100 Message-Id: <20180116151000.443-1-alban@kinvolk.io> Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk

==== The test I did was using a patched version of the memfs FUSE driver [2][3] and two very simple "hello-world" programs [5] (prog1 prints "hello world: 1" and prog2 prints "hello world: 2"). I copy prog1 and prog2 in the fuse-memfs mount point, execute them and check the sha1 hash in "/sys/kernel/security/ima/ascii_runtime_measurements". My patch on the memfs FUSE driver added a backdoor command to serve prog1 when the kernel asks for prog2 or vice-versa. In this way, I can exec prog1 and get it to print "hello world: 2" without ever replacing the file via the VFS, so the kernel is not aware of the change. The test was done using the branch "alban/fuse-flag-ima-nocache-v2" [4]. Step by step test procedure: 1. Mount the memfs FUSE using [3]: rm -f /tmp/memfs-switch* ; memfs -L DEBUG /mnt/memfs 2. Copy prog1 and prog2 using [5] cp prog1 /mnt/memfs/prog1 cp prog2 /mnt/memfs/prog2 3. Lookup the files and let the FUSE driver to keep the handles open: dd if=/mnt/memfs/prog1 bs=1 | (read -n 1 x ; sleep 3600 ) & dd if=/mnt/memfs/prog2 bs=1 | (read -n 1 x ; sleep 3600 ) & 4. Check the 2 programs work correctly: $ /mnt/memfs/prog1 hello world: 1 $ /mnt/memfs/prog2 hello world: 2 5. Check the measurements for prog1 and prog2: $ sudo cat /sys/kernel/security/ima/ascii_runtime_measurements \ | grep /mnt/memfs/prog 10 [...] ima-ng sha1:ac14c9268cd2[...] /mnt/memfs/prog1 10 [...] ima-ng sha1:799cb5d1e06d[...] /mnt/memfs/prog2 6. Use the backdoor command in my patched memfs to redirect file operations on file handle 3 to file handle 2: rm -f /tmp/memfs-switch* ; touch /tmp/memfs-switch-3-2 7. Check how the FUSE driver serves different content for the files: $ /mnt/memfs/prog1 hello world: 2 $ /mnt/memfs/prog2 hello world: 2 8. Check the measurements: sudo cat /sys/kernel/security/ima/ascii_runtime_measurements \ | grep /mnt/memfs/prog Without the patch, there are no new measurements, despite the FUSE driver having served different executables. With the patch, I can see additional measurements for prog1 and prog2 with the hashes reversed when the FUSE driver served the alternative content. ==== [1] https://www.spinics.net/lists/linux-integrity/msg00948.html [2] https://github.com/bbengfort/memfs [3] https://github.com/kinvolk/memfs/commits/alban/switch-files [4] https://github.com/kinvolk/linux/commits/alban/fuse-flag-ima-nocache-v2 [5] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0 Cc: linux-kernel@vger.kernel.org Cc: linux-integrity@vger.kernel.org Cc: linux-security-module@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org Cc: Miklos Szeredi <miklos@szeredi.hu> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Mimi Zohar <zohar@linux.vnet.ibm.com> Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com> Cc: James Morris <james.l.morris@oracle.com> Cc: "Serge E. Hallyn" <serge@hallyn.com> Cc: Seth Forshee <seth.forshee@canonical.com> Cc: Christoph Hellwig <hch@infradead.org> Tested-by: Dongsu Park <dongsu@kinvolk.io> Signed-off-by: Alban Crequy <alban@kinvolk.io> --- There was a previous attempt (unmerged) with a IMA option named "force" and using that option for FUSE filesystems. This patch uses a different approach so that the IMA subsystem does not need to know about FUSE. - https://www.spinics.net/lists/linux-integrity/msg00948.html - https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1584131.html Changes since v1: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1587390.html - include linux-fsdevel mailing list in cc - mark patch as RFC - based on next-integrity, without other unmerged FUSE / IMA patches --- fs/fuse/inode.c | 2 +- include/linux/fs.h | 1 + security/integrity/ima/ima_main.c | 24 ++++++++++++++++++++++-- 3 files changed, 24 insertions(+), 3 deletions(-) diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c index 624f18bbfd2b..5bf14289afba 100644 --- a/fs/fuse/inode.c +++ b/fs/fuse/inode.c @@ -1205,7 +1205,7 @@ static void fuse_kill_sb_anon(struct super_block *sb) static struct file_system_type fuse_fs_type = { .owner = THIS_MODULE, .name = "fuse", - .fs_flags = FS_HAS_SUBTYPE, + .fs_flags = FS_HAS_SUBTYPE | FS_NO_IMA_CACHE, .mount = fuse_mount, .kill_sb = fuse_kill_sb_anon, }; diff --git a/include/linux/fs.h b/include/linux/fs.h index 511fbaabf624..2bd7e73ebc2a 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -2075,6 +2075,7 @@ struct file_system_type { #define FS_BINARY_MOUNTDATA 2 #define FS_HAS_SUBTYPE 4 #define FS_USERNS_MOUNT 8 /* Can be mounted by userns root */ +#define FS_NO_IMA_CACHE 16 /* Force IMA to re-measure, re-appraise, re-audit files */ #define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move() during rename() internally. */ struct dentry *(*mount) (struct file_system_type *, int, const char *, void *); diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c index 6d78cb26784d..2ce4d69068a8 100644 --- a/security/integrity/ima/ima_main.c +++ b/security/integrity/ima/ima_main.c @@ -24,6 +24,7 @@ #include <linux/slab.h> #include <linux/xattr.h> #include <linux/ima.h> +#include <linux/fs.h> #include "ima.h" @@ -228,9 +229,28 @@ static int process_measurement(struct file *file, char *buf, loff_t size, IMA_APPRAISE_SUBMASK | IMA_APPRAISED_SUBMASK | IMA_ACTION_FLAGS); - if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags)) - /* reset all flags if ima_inode_setxattr was called */ + /* + * Reset the measure, appraise and audit cached flags either if: + * - ima_inode_setxattr was called, or + * - based on filesystem feature flag + * forcing the file to be re-evaluated. + */ + if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags)) { iint->flags &= ~IMA_DONE_MASK; + } else if (inode->i_sb->s_type->fs_flags & FS_NO_IMA_CACHE) { + if (action & IMA_MEASURE) { + iint->measured_pcrs = 0; + iint->flags &= + ~(IMA_COLLECTED | IMA_MEASURE | IMA_MEASURED); + } + if (action & IMA_APPRAISE) + iint->flags &= + ~(IMA_COLLECTED | IMA_APPRAISE | IMA_APPRAISED | + IMA_APPRAISE_SUBMASK | IMA_APPRAISED_SUBMASK); + if (action & IMA_AUDIT) + iint->flags &= + ~(IMA_COLLECTED | IMA_AUDIT | IMA_AUDITED); + } /* Determine if already appraised/measured based on bitmask * (IMA_MEASURE, IMA_MEASURED, IMA_XXXX_APPRAISE, IMA_XXXX_APPRAISED,

[RFC,v2] ima,fuse: introduce new fs flag FS_NO_IMA_CACHE

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