Re: [PATCH v7 10/14] securityfs: Extend securityfs with namespacing support

[Stefan Berger <stefanb@linux.ibm.com>]

On 12/16/21 08:40, Christian Brauner wrote:
> On Thu, Dec 16, 2021 at 12:43:19AM -0500, Stefan Berger wrote:
>> From: Stefan Berger <stefanb@linux.ibm.com>
>>
>> Extend 'securityfs' for support of IMA namespacing so that each
>> IMA (user) namespace can have its own front-end for showing the currently
>> active policy, the measurement list, number of violations and so on.
>>
>> Drop the addition dentry reference to enable simple cleanup of dentries
>> upon umount.
>>
>> Prevent mounting of an instance of securityfs in another user namespace
>> than it belongs to. Also, prevent accesses to directories when another
>> user namespace is active than the one that the instance of securityfs
>> belongs to.
>>
>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>> Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
>> ---
>>   security/inode.c | 37 ++++++++++++++++++++++++++++++++++---
>>   1 file changed, 34 insertions(+), 3 deletions(-)
>>
>> diff --git a/security/inode.c b/security/inode.c
>> index fee01ff4d831..a0d9f086e3d5 100644
>> --- a/security/inode.c
>> +++ b/security/inode.c
>> @@ -26,6 +26,29 @@
>>   static struct vfsmount *init_securityfs_mount;
>>   static int init_securityfs_mount_count;
>>   
>> +static int securityfs_permission(struct user_namespace *mnt_userns,
>> +				 struct inode *inode, int mask)
>> +{
>> +	int err;
>> +
>> +	err = generic_permission(&init_user_ns, inode, mask);
>> +	if (!err) {
>> +		if (inode->i_sb->s_user_ns != current_user_ns())
>> +			err = -EACCES;
> I really think the correct semantics is to grant all callers access
> whose user namespace is the same as or an ancestor of the securityfs
> userns. It's weird to deny access to callers who are located in an
> ancestor userns.

Ok, will be using current_in_userns() or the more explicit in_userns() 
for the check.


>
> For example, a privileged process on the host should be allowed to setns
> to the userns of an unprivileged container and inspect its securityfs
> instance.
>
> We're mostly interested to block such as scenarios where two sibling
> unprivileged containers are created in the initial userns and an fd
> proxy or something funnels a file descriptor from one sibling container
> to the another one and the receiving sibling container can use readdir()
> or openat() on this fd. (I'm not even convinced that this is actually a
> problem but stricter semantics at the beginning can't hurt. We can
> always relax this later.)