[RFC PATCH 2/4] ima: fail signature verification on unprivileged & untrusted filesystems

[Mimi Zohar]

On Wed, 2018-02-14 at 17:57 -0600, Eric W. Biederman wrote:
> Mimi Zohar <zohar at linux.vnet.ibm.com> writes:
> 
> > Files on untrusted filesystems, such as fuse, can change at any time,
> > making the measurement(s) and by extension signature verification
> > meaningless.
> >
> > FUSE can be mounted by unprivileged users either today with fusermount
> > installed with setuid, or soon with the upcoming patches to allow FUSE
> > mounts in a non-init user namespace.
> >
> > This patch always fails the file signature verification on unprivileged
> > and untrusted filesystems.  To also fail file signature verification on
> > privileged, untrusted filesystems requires a custom policy.
> >
> > (This patch is based on Alban Crequy's use of fs_flags and patch
> >  description.)
> 
> This would be much better done based on a flag in s_iflags and then the
> mounts that need this can set this.  That new flag can perhaps be called
> SB_I_IMA_FAIL.
> 
> Among other things that should allow the policy of when to set this to
> be set in fuse where it is obvious rather than in an magic location in
> IMA.

Using s_iflags instead of fs_flags is fine, but I'm not sure how this
affects the IMA policy.  This patch set assumes only unprivileged,
untrusted filesytems can automatically fail file signature
verification (2nd patch), as that hasn't yet been upstreamed and won't
break userspace.

Based on policy, IMA should additionally be able to fail the signature
verification for files on privileged, untrusted filesystems.

Mimi


> Eric
> 
> > Signed-off-by: Mimi Zohar <zohar at linux.vnet.ibm.com>
> > Cc: Miklos Szeredi <miklos at szeredi.hu>
> > Cc: Seth Forshee <seth.forshee at canonical.com>
> > Cc: Eric W. Biederman <ebiederm at xmission.com>
> > Cc: Dongsu Park <dongsu at kinvolk.io>
> > Cc: Alban Crequy <alban at kinvolk.io>
> > Cc: "Serge E. Hallyn" <serge at hallyn.com>
> > ---
> >  include/linux/fs.h                    |  1 +
> >  security/integrity/ima/ima_appraise.c | 10 +++++++++-
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 2a815560fda0..faffe4aab43d 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -2069,6 +2069,7 @@ 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_UNTRUSTED		16	/* Defined filesystem as untrusted */
> >  #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_appraise.c b/security/integrity/ima/ima_appraise.c
> > index f2803a40ff82..af8add31fe26 100644
> > --- a/security/integrity/ima/ima_appraise.c
> > +++ b/security/integrity/ima/ima_appraise.c
> > @@ -292,7 +292,14 @@ int ima_appraise_measurement(enum ima_hooks func,
> >  	}
> >  
> >  out:
> > -	if (status != INTEGRITY_PASS) {
> > +	/* Fail untrusted and unpriviliged filesystems (eg FUSE) */
> > +	if ((inode->i_sb->s_type->fs_flags & FS_UNTRUSTED) &&
> > +	    (inode->i_sb->s_user_ns != &init_user_ns)) {
> > +		status = INTEGRITY_FAIL;
> > +		cause = "untrusted-filesystem";
> > +		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
> > +				    op, cause, rc, 0);
> > +	} else if (status != INTEGRITY_PASS) {
> >  		if ((ima_appraise & IMA_APPRAISE_FIX) &&
> >  		    (!xattr_value ||
> >  		     xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
> > @@ -309,6 +316,7 @@ int ima_appraise_measurement(enum ima_hooks func,
> >  	} else {
> >  		ima_cache_flags(iint, func);
> >  	}
> > +
> >  	ima_set_cache_status(iint, func, status);
> >  	return status;
> >  }
> 

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