EVM verifies the existing 'security.evm' value, before allowing it
to be updated.  The EVM HMAC and the original file signatures contain
filesystem specific metadata (e.g. i_ino, i_generation and s_uuid).

This poses a challenge when transitioning from the lower backing file
to the upper backing file.

Until a complete solution is developed, disable EVM on overlayfs.

Changelog v2:
Addressed Amir's comments:
- Simplified security_inode_copy_up_xattr() return.
- Identified filesystems that don't support EVM based on a new SB_I flag.

Mimi Zohar (3):
  evm: don't copy up 'security.evm' xattr
  evm: add support to disable EVM on unsupported filesystems
  overlay: disable EVM

 fs/overlayfs/super.c              |  1 +
 include/linux/evm.h               |  6 +++++
 include/linux/fs.h                |  1 +
 security/integrity/evm/evm_main.c | 42 ++++++++++++++++++++++++++++++-
 security/security.c               |  2 +-
 5 files changed, 50 insertions(+), 2 deletions(-)

-- 
2.39.3

On Tue, Dec 19, 2023 at 12:52:03PM -0500, Mimi Zohar wrote:
> EVM verifies the existing 'security.evm' value, before allowing it
> to be updated.  The EVM HMAC and the original file signatures contain
> filesystem specific metadata (e.g. i_ino, i_generation and s_uuid).
> 
> This poses a challenge when transitioning from the lower backing file
> to the upper backing file.
> 
> Until a complete solution is developed, disable EVM on overlayfs.
> 
> Changelog v2:
> Addressed Amir's comments:
> - Simplified security_inode_copy_up_xattr() return.
> - Identified filesystems that don't support EVM based on a new SB_I flag.

We're wasting a flag for a single filesystem but we do have enough of
them left so I think this is ok,

Reviewed-by: Christian Brauner <brauner@kernel.org>

On Wed, 2023-12-20 at 13:35 +0100, Christian Brauner wrote:
> On Tue, Dec 19, 2023 at 12:52:03PM -0500, Mimi Zohar wrote:
> > EVM verifies the existing 'security.evm' value, before allowing it
> > to be updated.  The EVM HMAC and the original file signatures contain
> > filesystem specific metadata (e.g. i_ino, i_generation and s_uuid).
> > 
> > This poses a challenge when transitioning from the lower backing file
> > to the upper backing file.
> > 
> > Until a complete solution is developed, disable EVM on overlayfs.
> > 
> > Changelog v2:
> > Addressed Amir's comments:
> > - Simplified security_inode_copy_up_xattr() return.
> > - Identified filesystems that don't support EVM based on a new SB_I flag.
> 
> We're wasting a flag for a single filesystem but we do have enough of
> them left so I think this is ok,

Thanks, Christian.

> 
> Reviewed-by: Christian Brauner <brauner@kernel.org>