Re: [PATCH] ima,fuse: introduce new fs flag FS_NO_IMA_CACHE

[Alban Crequy]

On Tue, Jan 16, 2018 at 11:41 AM, Alban Crequy <alban.crequy@xxxxxxxxx> wrote:
> From: Alban Crequy <alban@xxxxxxxxxx>
>
> This patch forces files to be re-measured, re-appraised and re-audited
> on file systems with the feature flag FS_NO_IMA_CACHE. In that way,
> cached integrity results won't be used.
>
> For now, only FUSE filesystems use this flag. This is because the
> userspace FUSE process can change the underlying files at any times.
>
> This patch is based on the patch "ima: define a new policy option
> named force" by Mimi. [1]
>
> How to test this:
>
> ====
>
> 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" [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 7ac5aed52061cb09120e977c6d04ee5c7b11c371 ima-ng
> sha1:ac14c9268cd2811f7a5adea17b27d84f50e1122c /mnt/memfs/prog1
> 10 9acc17a9a32aec4a676b8f6558e17a3d6c9a78e6 ima-ng
> sha1:799cb5d1e06d5c37ae7a76ba25ecd1bd01476383 /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
> [5] https://github.com/kinvolk/fuse-userns-patches/commit/cf1f5750cab0
>
> Cc: linux-integrity@xxxxxxxxxxxxxxx
> Cc: linux-security-module@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> Cc: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> Cc: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Tested-by: Dongsu Park <dongsu@xxxxxxxxxx>
> Signed-off-by: Alban Crequy <alban@xxxxxxxxxx>
> ---
>  fs/fuse/inode.c                   |  2 +-
>  include/linux/fs.h                |  1 +
>  security/integrity/ima/ima_main.c | 11 +++++++----
>  3 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 8c98edee3628..b511e6469b0a 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1212,7 +1212,7 @@ 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_USERNS_MOUNT,
> +       .fs_flags       = FS_HAS_SUBTYPE | FS_USERNS_MOUNT | FS_NO_IMA_CACHE,

I just realised I should not have submitted this patch based on the
branch I am on because FS_USERNS_MOUNT is not there now, so this patch
does not apply cleanly on next-integrity at the moment. Sorry about
that.

>         .mount          = fuse_mount,
>         .kill_sb        = fuse_kill_sb_anon,
>  };
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index fce19c491970..88da6908a2b2 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 88af481502f7..e6e45ab15dfc 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"
>
> @@ -229,13 +230,15 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
>                                  IMA_ACTION_FLAGS);
>
>         /*
> -        * Reset the measure, appraise and audit cached flags either if
> -        * ima_inode_setxattr was called or based on policy, forcing
> -        * the file to be re-evaluated.
> +        * Reset the measure, appraise and audit cached flags either if:
> +        * - ima_inode_setxattr was called, or
> +        * - based on policy ("force"), or
> +        * - based on filesystem feature flag
> +        * forcing the file to be re-evaluated.
>          */

Now that I think about it, it's also possible to write this patch
without basing it on Mimi's patch "ima: define a new policy option
named force", which is not in next-integrity yet. Should I try that?

>         if (test_and_clear_bit(IMA_CHANGE_XATTR, &iint->atomic_flags)) {
>                 iint->flags &= ~IMA_DONE_MASK;
> -       } else if (action & IMA_FORCE) {
> +       } else if (action & IMA_FORCE || inode->i_sb->s_type->fs_flags & FS_NO_IMA_CACHE) {
>                 if (action & IMA_MEASURE) {
>                         iint->measured_pcrs = 0;
>                         iint->flags &=
> --
> 2.13.6
>