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

[Roberto Sassu]

On Tue, 2024-03-05 at 14:17 -0600, Seth Forshee (DigitalOcean) wrote:
> On Tue, Mar 05, 2024 at 06:11:45PM +0100, Roberto Sassu wrote:
> > On Tue, 2024-03-05 at 13:46 +0100, Roberto Sassu wrote:
> > > On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> > > > On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > > > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > > > > 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@xxxxxxxxxx>
> > > > > > > > > ---
> > > > > > > > >  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?
> > > > > > 
> > > > > > Uhm, I think it would not work without modifying
> > > > > > security_inode_need_killpriv() and the hook definition.
> > > > > > 
> > > > > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM would
> > > > > > not be invoked. We would need to continue the loop and let EVM know
> > > > > > what is the current return value. Then EVM can reject the change.
> > > > > > 
> > > > > > An alternative way would be to detect that actually we are setting the
> > > > > > same value for inode metadata, and maybe not returning 1 from
> > > > > > cap_inode_need_killpriv().
> > > > > > 
> > > > > > I would prefer the second, since EVM allows same value change and we
> > > > > > would have an exception if there are fscaps.
> > > > > > 
> > > > > > This solves only the case of portable signatures. We would need to
> > > > > > change cap_inode_need_killpriv() anyway to update the HMAC for mutable
> > > > > > files.
> > > > > 
> > > > > I see. In any case this sounds like a matter for a separate patch
> > > > > series.
> > > > 
> > > > Agreed.
> > > 
> > > Christian, how realistic is that we don't kill priv if we are setting
> > > the same owner?
> > > 
> > > Serge, would we be able to replace __vfs_removexattr() (or now
> > > vfs_get_fscaps_nosec()) with a security-equivalent alternative?
> > 
> > It seems it is not necessary.
> > 
> > security.capability removal occurs between evm_inode_setattr() and
> > evm_inode_post_setattr(), after the HMAC has been verified and before
> > the new HMAC is recalculated (without security.capability).
> > 
> > So, all good.
> > 
> > Christian, Seth, I pushed the kernel and the updated tests (all patches
> > are WIP):
> > 
> > https://github.com/robertosassu/linux/commits/evm-fscaps-v2/
> > 
> > https://github.com/robertosassu/ima-evm-utils/commits/evm-fscaps-v2/
> > 
> > 
> > The tests are passing:
> > 
> > https://github.com/robertosassu/ima-evm-utils/actions/runs/8159877004/job/22305521359
> 
> Thanks! I probably won't be able to take them exactly as-is due to other
> changes for the next version (rebasing onto the changes to make IMA and
> EVM LSMs, forbidding xattr handlers entirely for fscaps), but they will
> serve as a good road map for what needs to happen.

Welcome. Yes, both ima_inode_set_fscaps() and ima_inode_remove_fscaps()
will be registered as LSM hooks in ima_appraise.c. It will be probably
straightforward for you to make those changes, but if you have any
question, let me know.

Roberto