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Date: Wed, 24 Jan 2018 11:52:34 -0600
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Alban Crequy <alban.crequy@...il.com>
Cc: alban@...volk.io, dongsu@...volk.io, iago@...volk.io,
linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
linux-security-module@...r.kernel.org,
linux-fsdevel@...r.kernel.org, miklos@...redi.hu,
viro@...iv.linux.org.uk, zohar@...ux.vnet.ibm.com,
dmitry.kasatkin@...il.com, james.l.morris@...cle.com,
serge@...lyn.com, seth.forshee@...onical.com, hch@...radead.org
Subject: Re: [RFC PATCH v3 2/2] ima: force re-appraisal on filesystems with
FS_IMA_NO_CACHE
Quoting Alban Crequy (alban.crequy@...il.com):
> From: Alban Crequy <alban@...volk.io>
>
> This patch forces files to be re-measured, re-appraised and re-audited
> on file systems with the feature flag FS_IMA_NO_CACHE. In that way,
> cached integrity results won't be used.
>
> How to test this:
>
> The test I did was using a patched version of the memfs FUSE driver
> [1][2] and two very simple "hello-world" programs [4] (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-v3" [3].
>
> Step by step test procedure:
>
> 1. Mount the memfs FUSE using [2]:
> rm -f /tmp/memfs-switch* ; memfs -L DEBUG /mnt/memfs
>
> 2. Copy prog1 and prog2 using [4]
> 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://github.com/bbengfort/memfs
> [2] https://github.com/kinvolk/memfs/commits/alban/switch-files
> [3] https://github.com/kinvolk/linux/commits/alban/fuse-flag-ima-nocache-v3
> [4] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0
>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-integrity@...r.kernel.org
> Cc: linux-security-module@...r.kernel.org
> Cc: linux-fsdevel@...r.kernel.org
> Cc: Miklos Szeredi <miklos@...redi.hu>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> Cc: Dmitry Kasatkin <dmitry.kasatkin@...il.com>
> Cc: James Morris <james.l.morris@...cle.com>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
Acked-by: Serge Hallyn <serge@...lyn.com>
to both.
> Cc: Seth Forshee <seth.forshee@...onical.com>
> Cc: Christoph Hellwig <hch@...radead.org>
> Tested-by: Dongsu Park <dongsu@...volk.io>
> Signed-off-by: Alban Crequy <alban@...volk.io>
> ---
> security/integrity/ima/ima_main.c | 24 ++++++++++++++++++++++--
> 1 file changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 6d78cb26784d..8870a7bbe9b9 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_IMA_NO_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,
> --
> 2.13.6
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