[RFC,1/2] security, capabilities: create CAP_TRUSTED

Message ID 20171021134558.21195-1-nicolas@belouin.fr
State New
Delegated to: Richard Weinberger
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Series
  • [RFC,1/2] security, capabilities: create CAP_TRUSTED
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Commit Message

Nicolas Belouin Oct. 21, 2017, 1:45 p.m.
with CAP_SYS_ADMIN being bloated, the usefulness of using it to
flag a process to be entrusted for e.g reading and writing trusted
xattr is near zero.
CAP_TRUSTED aims to provide userland with a way to mark a process as
entrusted to do specific (not specially admin-centered) actions. It
would for example allow a process to red/write the trusted xattrs.

Signed-off-by: Nicolas Belouin <nicolas@belouin.fr>
---
 include/uapi/linux/capability.h     | 6 +++++-
 security/selinux/include/classmap.h | 5 +++--
 2 files changed, 8 insertions(+), 3 deletions(-)

Comments

Serge E. Hallyn Oct. 21, 2017, 4:03 p.m. | #1
Quoting Nicolas Belouin (nicolas@belouin.fr):
> with CAP_SYS_ADMIN being bloated, the usefulness of using it to
> flag a process to be entrusted for e.g reading and writing trusted
> xattr is near zero.
> CAP_TRUSTED aims to provide userland with a way to mark a process as
> entrusted to do specific (not specially admin-centered) actions. It
> would for example allow a process to red/write the trusted xattrs.

You say "for example".  Are you intending to add more uses?  If so, what
are they?  If not, how about renaming it CAP_TRUSTED_XATTR?

What all does allowing writes to trusted xattrs give you?  There are
the overlayfs whiteouts, what else?
Casey Schaufler Oct. 21, 2017, 5:25 p.m. | #2
On 10/21/2017 6:45 AM, Nicolas Belouin wrote:
> with CAP_SYS_ADMIN being bloated, the usefulness of using it to
> flag a process to be entrusted for e.g reading and writing trusted
> xattr is near zero.
> CAP_TRUSTED aims to provide userland with a way to mark a process as
> entrusted to do specific (not specially admin-centered) actions. It
> would for example allow a process to red/write the trusted xattrs.

Please explain how this is different from CAP_MAC_ADMIN in
any existing use case. If it is significantly different, how
would the two interact?

> Signed-off-by: Nicolas Belouin <nicolas@belouin.fr>
> ---
>  include/uapi/linux/capability.h     | 6 +++++-
>  security/selinux/include/classmap.h | 5 +++--
>  2 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index ce230aa6d928..27e457b93c84 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -369,7 +369,11 @@ struct vfs_ns_cap_data {
>  
>  #define CAP_SYS_MOUNT		38
>  
> -#define CAP_LAST_CAP         CAP_SYS_MOUNT
> +/* Allow read/write trusted xattr */
> +
> +#define CAP_TRUSTED		39
> +
> +#define CAP_LAST_CAP         CAP_TRUSTED
>  
>  #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>  
> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> index a873dce97fd5..f5dc8e109f5a 100644
> --- a/security/selinux/include/classmap.h
> +++ b/security/selinux/include/classmap.h
> @@ -24,9 +24,10 @@
>  	    "audit_control", "setfcap"
>  
>  #define COMMON_CAP2_PERMS  "mac_override", "mac_admin", "syslog", \
> -		"wake_alarm", "block_suspend", "audit_read", "sys_mount"
> +		"wake_alarm", "block_suspend", "audit_read", "sys_mount", \
> +		"trusted"
>  
> -#if CAP_LAST_CAP > CAP_SYS_MOUNT
> +#if CAP_LAST_CAP > CAP_TRUSTED
>  #error New capability defined, please update COMMON_CAP2_PERMS.
>  #endif
>
Nicolas Belouin Oct. 21, 2017, 7:04 p.m. | #3
<james.l.morris@oracle.com>,linux-ext4@vger.kernel.org,linux-kernel@vger.kernel.org,linux-f2fs-devel@lists.sourceforge.net,linux-fsdevel@vger.kernel.org,linux-mtd@lists.infradead.org,jfs-discussion@lists.sourceforge.net,ocfs2-devel@oss.oracle.com,linux-unionfs@vger.kernel.org,reiserfs-devel@vger.kernel.org,linux-security-module@vger.kernel.org,selinux@tycho.nsa.gov,linux-api@vger.kernel.org,kernel-hardening@lists.openwall.com
From: Nicolas Belouin <nicolas@belouin.fr>
Message-ID: <E5A1BDA4-A309-4118-84D5-72780F619EBA@belouin.fr>



On October 21, 2017 7:25:21 PM GMT+02:00, Casey Schaufler <casey@schaufler-ca.com> wrote:
>On 10/21/2017 6:45 AM, Nicolas Belouin wrote:
>> with CAP_SYS_ADMIN being bloated, the usefulness of using it to
>> flag a process to be entrusted for e.g reading and writing trusted
>> xattr is near zero.
>> CAP_TRUSTED aims to provide userland with a way to mark a process as
>> entrusted to do specific (not specially admin-centered) actions. It
>> would for example allow a process to red/write the trusted xattrs.
>
>Please explain how this is different from CAP_MAC_ADMIN in
>any existing use case. If it is significantly different, how
>would the two interact?

From my point of view, CAP_MAC_ADMIN allows one to read/write security xattrs, those are meant to describe security policies. As far as I know of, trusted xattrs are intended for a privileged process to read or write arbitrary data. I don't have any real world example in mind that use trusted xattrs, but I'll try to find one.

>
>> Signed-off-by: Nicolas Belouin <nicolas@belouin.fr>
>> ---
>>  include/uapi/linux/capability.h     | 6 +++++-
>>  security/selinux/include/classmap.h | 5 +++--
>>  2 files changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/uapi/linux/capability.h
>b/include/uapi/linux/capability.h
>> index ce230aa6d928..27e457b93c84 100644
>> --- a/include/uapi/linux/capability.h
>> +++ b/include/uapi/linux/capability.h
>> @@ -369,7 +369,11 @@ struct vfs_ns_cap_data {
>>  
>>  #define CAP_SYS_MOUNT		38
>>  
>> -#define CAP_LAST_CAP         CAP_SYS_MOUNT
>> +/* Allow read/write trusted xattr */
>> +
>> +#define CAP_TRUSTED		39
>> +
>> +#define CAP_LAST_CAP         CAP_TRUSTED
>>  
>>  #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>>  
>> diff --git a/security/selinux/include/classmap.h
>b/security/selinux/include/classmap.h
>> index a873dce97fd5..f5dc8e109f5a 100644
>> --- a/security/selinux/include/classmap.h
>> +++ b/security/selinux/include/classmap.h
>> @@ -24,9 +24,10 @@
>>  	    "audit_control", "setfcap"
>>  
>>  #define COMMON_CAP2_PERMS  "mac_override", "mac_admin", "syslog", \
>> -		"wake_alarm", "block_suspend", "audit_read", "sys_mount"
>> +		"wake_alarm", "block_suspend", "audit_read", "sys_mount", \
>> +		"trusted"
>>  
>> -#if CAP_LAST_CAP > CAP_SYS_MOUNT
>> +#if CAP_LAST_CAP > CAP_TRUSTED
>>  #error New capability defined, please update COMMON_CAP2_PERMS.
>>  #endif
>>  

Nicolas
Nicolas Belouin Oct. 21, 2017, 7:09 p.m. | #4
<james.l.morris@oracle.com>,linux-ext4@vger.kernel.org,linux-kernel@vger.kernel.org,linux-f2fs-devel@lists.sourceforge.net,linux-fsdevel@vger.kernel.org,linux-mtd@lists.infradead.org,jfs-discussion@lists.sourceforge.net,ocfs2-devel@oss.oracle.com,linux-unionfs@vger.kernel.org,reiserfs-devel@vger.kernel.org,linux-security-module@vger.kernel.org,selinux@tycho.nsa.gov,linux-api@vger.kernel.org,kernel-hardening@lists.openwall.com
From: Nicolas Belouin <nicolas@belouin.fr>
Message-ID: <99179B10-4EAE-4FAB-9D14-B885156261B3@belouin.fr>



On October 21, 2017 6:03:02 PM GMT+02:00, "Serge E. Hallyn" <serge@hallyn.com> wrote:
>Quoting Nicolas Belouin (nicolas@belouin.fr):
>> with CAP_SYS_ADMIN being bloated, the usefulness of using it to
>> flag a process to be entrusted for e.g reading and writing trusted
>> xattr is near zero.
>> CAP_TRUSTED aims to provide userland with a way to mark a process as
>> entrusted to do specific (not specially admin-centered) actions. It
>> would for example allow a process to red/write the trusted xattrs.
>
>You say "for example".  Are you intending to add more uses?  If so,
>what
>are they?  If not, how about renaming it CAP_TRUSTED_XATTR?
>

I don't see any other use for now, but I don't want it to be too narrow and non usable in a similar context in the future. So I believe the underlying purpose of marking a process as "trusted" (even if for now it only means rw permission on trusted xattr) is more meaningful.

>What all does allowing writes to trusted xattrs give you?  There are
>the overlayfs whiteouts, what else?

Nicolas

Patch

diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
index ce230aa6d928..27e457b93c84 100644
--- a/include/uapi/linux/capability.h
+++ b/include/uapi/linux/capability.h
@@ -369,7 +369,11 @@  struct vfs_ns_cap_data {
 
 #define CAP_SYS_MOUNT		38
 
-#define CAP_LAST_CAP         CAP_SYS_MOUNT
+/* Allow read/write trusted xattr */
+
+#define CAP_TRUSTED		39
+
+#define CAP_LAST_CAP         CAP_TRUSTED
 
 #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
 
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index a873dce97fd5..f5dc8e109f5a 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -24,9 +24,10 @@ 
 	    "audit_control", "setfcap"
 
 #define COMMON_CAP2_PERMS  "mac_override", "mac_admin", "syslog", \
-		"wake_alarm", "block_suspend", "audit_read", "sys_mount"
+		"wake_alarm", "block_suspend", "audit_read", "sys_mount", \
+		"trusted"
 
-#if CAP_LAST_CAP > CAP_SYS_MOUNT
+#if CAP_LAST_CAP > CAP_TRUSTED
 #error New capability defined, please update COMMON_CAP2_PERMS.
 #endif