Re: [PATCH 1/5] security: allow finer granularity in permitting copy-up of security xattrs

[Stefan Berger]

On 1/31/24 09:25, Christian Brauner wrote:
> On Wed, Jan 31, 2024 at 03:25:29PM +0200, Amir Goldstein wrote:
> > On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb@xxxxxxxxxxxxx> wrote:
> > > Copying up xattrs is solely based on the security xattr name. For finer
> > > granularity add a dentry parameter to the security_inode_copy_up_xattr
> > > hook definition, allowing decisions to be based on the xattr content as
> > > well.
> > >
> > > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > > ---
> > >   fs/overlayfs/copy_up.c            | 2 +-
> > >   include/linux/evm.h               | 2 +-
> > >   include/linux/lsm_hook_defs.h     | 3 ++-
> > >   include/linux/security.h          | 4 ++--
> > >   security/integrity/evm/evm_main.c | 2 +-
> > >   security/security.c               | 7 ++++---
> > >   security/selinux/hooks.c          | 2 +-
> > >   security/smack/smack_lsm.c        | 2 +-
> > >   8 files changed, 13 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > > index b8e25ca51016..bd9ddcefb7a7 100644
> > > --- a/fs/overlayfs/copy_up.c
> > > +++ b/fs/overlayfs/copy_up.c
> > > @@ -114,7 +114,7 @@ int ovl_copy_xattr(struct super_block *sb, const struct path *oldpath, struct de
> > >                  if (ovl_is_private_xattr(sb, name))
> > >                          continue;
> > >
> > > -               error = security_inode_copy_up_xattr(name);
> > > +               error = security_inode_copy_up_xattr(old, name);
> >
> > What do you think about:
> >
> >                       error = security_inode_copy_up_xattr(name, NULL, 0);
> >
> > and then later...
> >
> >                       error = security_inode_copy_up_xattr(name, value, size);
> >
> > I am asking because overlayfs uses mnt_idmap(path->mnt) and you
> > have used nop_mnt_idmap inside evm hook.
> > this does not look right to me?
>
> So it's relevant if they interact with xattrs that care about the
> idmapping and that's POSIX ACLs and fscaps. And only if they perform
> permission checks such as posix_acl_update_mode() or something. IOW, it
> depends on what exactly EVM is doing.

In 2/5 we are reading the value of security.evm to look at its contents.

> IIRC, I already added custom security_*() hooks specifically for POSIX
> ACLs as they can't be retrieved through vfs_xattr*() helpers anymore.