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 >
