Re: [PATCH v2 24/25] commoncap: use vfs fscaps interfaces

Seth Forshee (DigitalOcean)
2024-03-04T15:31:58Z

On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wrote:
> > Use the vfs interfaces for fetching file capabilities for killpriv
> > checks and from get_vfs_caps_from_disk(). While there, update the
> > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > from vfs_get_fscaps_nosec().
> >
> > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
> > ---
> > security/commoncap.c | 30 +++++++++++++-----------------
> > 1 file changed, 13 insertions(+), 17 deletions(-)
> >
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index a0ff7e6092e0..751bb26a06a6 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > */
> > int cap_inode_need_killpriv(struct dentry *dentry)
> > {
> > - struct inode *inode = d_backing_inode(dentry);
> > + struct vfs_caps caps;
> > int error;
> >
> > - error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > - return error > 0;
> > + /* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > + error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > + return error == 0;
> > }
> >
> > /**
> > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> > {
> > int error;
> >
> > - error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > + error = vfs_remove_fscaps_nosec(idmap, dentry);
>
> Uhm, I see that the change is logically correct... but the original
> code was not correct, since the EVM post hook is not called (thus the
> HMAC is broken, or an xattr change is allowed on a portable signature
> which should be not).
>
> For completeness, the xattr change on a portable signature should not
> happen in the first place, so cap_inode_killpriv() would not be called.
> However, since EVM allows same value change, we are here.

I really don't understand EVM that well and am pretty hesitant to try an
change any of the logic around it. But I'll hazard a thought: should EVM
have a inode_need_killpriv hook which returns an error in this
situation?