[PATCH 0/5] evm: Prepare for moving to the LSM infrastructure

Roberto Sassu roberto.sassu at huawei.com
Fri Apr 16 16:37:10 UTC 2021

> From: Casey Schaufler [mailto:casey at schaufler-ca.com]
> Sent: Thursday, April 15, 2021 10:44 PM
> On 4/15/2021 3:04 AM, Roberto Sassu wrote:
> > This patch set depends on:
> >
> > https://lore.kernel.org/linux-integrity/20210409114313.4073-1-
> roberto.sassu at huawei.com/
> > https://lore.kernel.org/linux-integrity/20210407105252.30721-1-
> roberto.sassu at huawei.com/
> >
> > One of the challenges that must be tackled to move IMA and EVM to the LSM
> > infrastructure is to ensure that EVM is capable to correctly handle
> > multiple stacked LSMs providing an xattr at file creation. At the moment,
> > there are few issues that would prevent a correct integration. This patch
> > set aims at solving them.
> >
> > From the LSM infrastructure side, the LSM stacking feature added the
> > possibility of registering multiple implementations of the security hooks,
> > that are called sequentially whenever someone calls the corresponding
> > security hook. However, security_inode_init_security() and
> > security_old_inode_init_security() are currently limited to support one
> > xattr provided by LSM and one by EVM.
> That is correct. At present the only two modules that provide extended
> attributes are SELinux and Smack. The LSM infrastructure requires more
> change, including change to security_inode_init_security(), before those
> modules can be used together.

One of the goals of this patch set is to solve the specific problem
of security_inode_init_security(), when arbitrary LSMs are added
to the LSM infrastructure. Given that some problems have
been already identified, and will arise when a new LSM
providing an implementation for the inode_init_security hook
will be added to the LSM infrastructure, it seems a good idea
fixing them. We could discuss about the solution, if there is
a better approach.

> > In addition, using the call_int_hook() macro causes some issues. According
> > to the documentation in include/linux/lsm_hooks.h, it is a legitimate case
> > that an LSM returns -EOPNOTSUPP when it does not want to provide an xattr.
> > However, the loop defined in the macro would stop calling subsequent LSMs
> > if that happens. In the case of security_old_inode_init_security(), using
> > the macro would also cause a memory leak due to replacing the *value
> > pointer, if multiple LSMs provide an xattr.
> As there is no case where there will be multiple providers of hooks for
> inode_init_security this isn't an issue.

I could skip the patches that are not required to support
multiple LSMs registering to the inode_init_security hook
and just do the EVM changes (see below for the motivation).

> > From EVM side, the first operation to be done is to change the definition
> > of evm_inode_init_security() to be compatible with the security hook
> > definition. Unfortunately, the current definition does not provide enough
> > information for EVM, as it must have visibility of all xattrs provided by
> > LSMs to correctly calculate the HMAC. This patch set changes the security
> > hook definition by adding the full array of xattr as a parameter.
> Why do you want to call evm_inode_init_security() as a regular LSM hook?
> Except for the names evm_inode_init_security() and
> selinux_inode_init_security()
> have nothing in common. They do very different things and require different
> data, as comes out in the patches.

I thought that it would be more clean if all hooks are registered
to the LSM infrastructure. Otherwise, it could happen that some
hooks are still executed even if the LSM is not active, from the
perspective of the LSM infrastructure.

evm_inode_init_security() is still a provider of xattrs, like the
other LSMs, just it requires an extra parameter to calculate
the HMAC.

> There are evm functions that could be implemented as LSM hooks. I don't think
> this is one of them. There's no point in going overboard.

IMA and EVM both use a cache to store the integrity verification,
which is currently not managed by the LSM infrastructure but
by an ad-hoc mechanism implemented with an rbtree.

One of the benefits of defining both IMA and EVM as an LSM
is that we can switch from this ad-hoc mechanism to the one
implemented for the LSM infrastructure, with a search in
constant time. Given that evm_inode_init_security() would
update the integrity status (xattrs are good at inode creation
time), I would add it as well to the LSM infrastructure.

One additional motivation for defining EVM as an LSM is that
it would solve one of the EVM limitations that affects its
usability: partial copy of xattrs (e.g. by cp and tar) would not
work when an HMAC key is loaded because, since EVM in
the post set/removexattr hook does not know the status
of the last integrity verification, it has to deny the permission
to perform the xattr operation, to avoid that the HMAC is
calculated on corrupted xattrs. Having the status in the
per-inode blob would solve this issue more efficiently than
adding a cache for each verified inode in the rbtree.

Would you see this as an useful modification?



Managing Director: Li Peng, Li Jian, Shi Yanli

> > Secondly, EVM must know how many elements are in the xattr array. It
> seems
> > that it is not necessary to add another parameter, as all filesystems that
> > define an initxattr function, expect that the last element of the array
> is
> > one with the name field set to NULL. EVM reuses the same assumption.
> >
> > This patch set has been tested by introducing several instances of a
> > TestLSM (some providing an xattr, some not, one with a wrong
> implementation
> > to see how the LSM infrastructure handles it). The patch is not included
> > in this set but it is available here:
> >
> >
> https://github.com/robertosassu/linux/commit/0370ff0fbc16e5d63489836a95
> 8e65d697f956db
> >
> > The test, added to ima-evm-utils, is available here:
> >
> > https://github.com/robertosassu/ima-evm-utils/blob/evm-multiple-lsms-v1-
> devel-v1/tests/evm_multiple_lsms.test
> >
> > The test takes a UML kernel built by Travis and launches it several times,
> > each time with a different combination of LSMs. After boot, it first checks
> > that there is an xattr for each LSM providing it, and then calculates the
> > HMAC in user space and compares it with the HMAC calculated by EVM in
> > kernel space.
> >
> > A test report can be obtained here:
> >
> > https://www.travis-ci.com/github/robertosassu/ima-evm-
> utils/jobs/498699540
> >
> > Lastly, running the test on reiserfs to check
> > security_old_inode_init_security(), some issues have been discovered: a
> > free of xattr->name which is not correct after commit 9548906b2bb7 ('xattr:
> > Constify ->name member of "struct xattr"'), and a misalignment with
> > security_inode_init_security() (the old version expects the full xattr name
> > with the security. prefix, the new version just the suffix). The last issue
> > has not been fixed yet.
> >
> > Roberto Sassu (5):
> >   xattr: Complete constify ->name member of "struct xattr"
> >   security: Support multiple LSMs implementing the inode_init_security
> >     hook
> >   security: Pass xattrs allocated by LSMs to the inode_init_security
> >     hook
> >   evm: Align evm_inode_init_security() definition with LSM
> >     infrastructure
> >   evm: Support multiple LSMs providing an xattr
> >
> >  fs/reiserfs/xattr_security.c        |  2 -
> >  include/linux/evm.h                 | 21 ++++---
> >  include/linux/lsm_hook_defs.h       |  2 +-
> >  include/linux/lsm_hooks.h           |  5 +-
> >  security/integrity/evm/evm.h        |  2 +
> >  security/integrity/evm/evm_crypto.c |  9 ++-
> >  security/integrity/evm/evm_main.c   | 35 +++++++----
> >  security/security.c                 | 95 +++++++++++++++++++++++------
> >  security/selinux/hooks.c            |  3 +-
> >  security/smack/smack_lsm.c          |  4 +-
> >  10 files changed, 135 insertions(+), 43 deletions(-)
> >

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