Quoting Alban Crequy (alban.crequy@xxxxxxxxx): > 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_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@xxxxxxxxxxxxxxx > Cc: linux-integrity@xxxxxxxxxxxxxxx > Cc: linux-security-module@xxxxxxxxxxxxxxx > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > Cc: Miklos Szeredi <miklos@xxxxxxxxxx> > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> > Cc: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> > Cc: Dmitry Kasatkin <dmitry.kasatkin@xxxxxxxxx> > Cc: James Morris <james.l.morris@xxxxxxxxxx> > Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx> Acked-by: Serge Hallyn <serge@xxxxxxxxxx> to both. > Cc: Seth Forshee <seth.forshee@xxxxxxxxxxxxx> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> > Tested-by: Dongsu Park <dongsu@xxxxxxxxxx> > Signed-off-by: Alban Crequy <alban@xxxxxxxxxx> > --- > 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
