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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: References: <158932282880.2885325.2688622278854566047.stgit@warthog.procyon.org.uk> <3999877.1589475539@warthog.procyon.org.uk> To: Stephen Smalley Cc: dhowells@redhat.com, Jarkko Sakkinen , Paul Moore , Casey Schaufler , keyrings@vger.kernel.org, SElinux list , LSM List , linux-kernel Subject: Re: [PATCH] keys: Move permissions checking decisions into the checking code MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <196729.1589561109.1@warthog.procyon.org.uk> Content-Transfer-Encoding: quoted-printable Date: Fri, 15 May 2020 17:45:09 +0100 Message-ID: <196730.1589561109@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: Stephen Smalley wrote: > > (1) KEY_FLAG_KEEP in key->flags - The key may not be deleted and/= or things > > may not be removed from the keyring. > = > Why can't they be deleted / removed? They can't ever be deleted or > removed or for some period of time? This is only settable internally to keep special keys, such as the blackli= st loaded from the EFI BIOS, from being removed. > > (2) KEY_FLAG_ROOT_CAN_CLEAR in key->flags - The keyring can be cl= eared by > > CAP_SYS_ADMIN. > = > Why do some keyrings get this flag and others do not? Under what > conditions? Why is CAP_SYS_ADMIN the right capability for this? > = > > (3) KEY_FLAG_ROOT_CAN_INVAL in key->flags - The key can be invali= dated by > > CAP_SYS_ADMIN. > = > Ditto. So that the sysadmin can clear, say, the NFS idmapper keyring or invalidat= e DNS lookup keys. > > (4) An appropriate auth token being set in cred->request_key_auth= that > > gives a process transient permission to view and instantiate = a key. > > This is used by the kernel to delegate instantiation to users= pace. > = > Is this ever allowed across different credentials? The kernel upcalls by spawning a daemon. I want to change this as it's no= t compatible with containers since namespaces make this problematic. > When? The request_key() system call will do this. The normal use case is someth= ing like the AFS filesystem asking for a key so that it can do an operation. = The possibility exists for the kernel to upcall, say, to something that does a= klog on behalf of the user - but aklog in turn needs to get the TGT out of the keyrings. > Why? Is there a check between the different credentials before the > auth token is created? No. I don't even know what the target creds will necessarily be at this point. > > Note that this requires some tweaks to the testsuite as some of th= e > > error codes change. > = > Which testsuite? keyring or selinux or both? The keyring testsuite. No idea about the SELinux one. > What error codes change (from what to what)? Does this constitute an AB= I > change? The following: (1) Passing the wrong type of key to KEYCTL_DH_COMPUTE now gets you EOPNOTSUPP rather than ENOKEY. This is now as documented in the manu= al page. (2) Passing key ID 0 or an invalid negative key ID to KEYCTL_DH_COMPUTE n= ow gets you EINVAL rather than ENOKEY. (3) Passing key ID 0 or an invalid negative key ID to KEYCTL_READ now get= s you EINVAL rather than ENOKEY. Technically, it consistutes an ABI change, I suppose, but I think it is probably sufficiently minor. Or maybe on (2) and (3) I should go the other way. You get ENOKEY for inv= alid key IDs (such as 0 or unsupported negative ones) across all callers of lookup_user_key(). This would at least by consistent with the manual page= s. > I like moving more of the permission checking logic into the security > modules and giving them greater visibility and control. That said, I > am somewhat concerned by the scale of this change, by the extent to > which you are exposing keyring internals inside the security modules, > and by the extent to which logic is getting duplicated in each > security module. It's what you asked for. Now, I don't know if the LSM needs to know that the main keyutils permissi= ons checker invoked an override. At least one of the overrides will have gone through the LSM anyway when capable() was called. > I'd suggest a more incremental approach, e.g. start with just the enum > patch, then migrate the easy cases, then consider the more complicated > cases. And possibly we need multiple different security hooks for the > keyring subsystem that are more specialized for the complicated cases. = If > we authorize the delegation up front, we don't need to check it later. I'll consider it. But I really need to get what I'm going to include in t= he middle of the notifications patchset sorted now - or risk the notification= s and fsinfo patchsets getting bumped again. Maybe what's needed is a pair of hooks whereby the call to capable() is replaced with LSM hook specifically to ask about the overrides: security_key_use_sysadmin_override(key, cred); security_key_use_construction_override(key, cred); And/or a hook to ask whether the process is allowed to do the request_key(= ) call that they want: security_request_key(struct key_type *type, const char *description, struct key_tag *domain_tag, const void *callout_info, size_t callout_len, void *aux); I don't really want to do a "can the kernel delegate to process X?" hook j= ust at the moment, since I want to change/extend that code and I don't want to commit to any particular security information being present yet. I can go back to the enum patch for the moment if you and Casey can put up with that for the moment? David