Re: [PATCH 1/1 V2] mqueue: Implment generic xattr support

[Paul Moore]

On Wed, Nov 9, 2016 at 11:25 AM, David Graziano
<david.graziano@xxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Nov 7, 2016 at 4:23 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>> On Mon, Nov 7, 2016 at 3:46 PM, David Graziano
>> <david.graziano@xxxxxxxxxxxxxxxxxxx> wrote:
>>> This patch adds support for generic extended attributes within the
>>> POSIX message queues filesystem and setting them by consulting the LSM.
>>> This is needed so that the security.selinux extended attribute can be
>>> set via a SELinux named type transition on file inodes created within
>>> the filesystem. The implementation and LSM call back function are based
>>> off tmpfs/shmem.
>>>
>>> Signed-off-by: David Graziano <david.graziano@xxxxxxxxxxxxxxxxxxx>
>>> ---
>>>  ipc/mqueue.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 46 insertions(+)
>>
>> Hi David,
>>
>> At first glance this looks reasonable to me, I just have a two
>> questions/comments:
>>
>> * Can you explain your current need for this functionality?  For
>> example, what are you trying to do that is made easier by allowing
>> greater message queue labeling flexibility?  This helps put things in
>> context and helps people review and comment on your patch.
>>
>> * How have you tested this?  While this patch is not SELinux specific,
>> I think adding a test to the selinux-testsuite[1] would be worthwhile.
>> The other LSM maintainers may suggest something similar if they have
>> an established public testsuite.
>>
>> [1] https://github.com/SELinuxProject/selinux-testsuite
>
> Hi Paul,
>
> I needed to write a selinux policy for a set of custom applications that use
> POSIX message queues for their IPC. The queues are created by one
> application and we needed a way for selinux to enforce which of the other
> apps are able to read/write to each individual queue. Uniquely labeling them
> based on the app that created them and the file name seemed to be our best
> solution since itâs an embedded system and we donât have restorecond to
> handle any relabeling.

In the future putting things like the above in the patch description
can be helpful.  In other words, instead of simply saying this allows
you to better control the labels assigned to message queues, you could
expand upon it by saying that this patch allows you to better control
which applications have access to a given queue.  Yes, I realize that
is implied by better control over the labels, but being explicit is
rarely a bad thing when it comes to patch descriptions.

I've never rejected a patch for a description that was too lengthy,
but I have rejected patches that need better descriptions ;)

> To test this patch I used both a selinux enabled, buildroot based qemu target
> with a customized selinux policy and test C app to create the mqueues. I also
> tested with our real apps and selinux policy on our target hardware. I can
> certainly look at adding a test to the selinux-testsuite if that would
> be helpful.

Please do.  I've been requiring tests for all new SELinux
functionality lately; this isn't strictly a SELinux patch but I think
it is a good practice regardless.

>>> diff --git a/ipc/mqueue.c b/ipc/mqueue.c
>>> index 0b13ace..512a546 100644
>>> --- a/ipc/mqueue.c
>>> +++ b/ipc/mqueue.c
>>> @@ -35,6 +35,7 @@
>>>  #include <linux/ipc_namespace.h>
>>>  #include <linux/user_namespace.h>
>>>  #include <linux/slab.h>
>>> +#include <linux/xattr.h>
>>>
>>>  #include <net/sock.h>
>>>  #include "util.h"
>>> @@ -70,6 +71,7 @@ struct mqueue_inode_info {
>>>         struct rb_root msg_tree;
>>>         struct posix_msg_tree_node *node_cache;
>>>         struct mq_attr attr;
>>> +       struct simple_xattrs xattrs;    /* list of xattrs */
>>>
>>>         struct sigevent notify;
>>>         struct pid *notify_owner;
>>> @@ -254,6 +256,7 @@ static struct inode *mqueue_get_inode(struct super_block *sb,
>>>                         info->attr.mq_maxmsg = attr->mq_maxmsg;
>>>                         info->attr.mq_msgsize = attr->mq_msgsize;
>>>                 }
>>> +               simple_xattrs_init(&info->xattrs);
>>>                 /*
>>>                  * We used to allocate a static array of pointers and account
>>>                  * the size of that array as well as one msg_msg struct per
>>> @@ -413,6 +416,41 @@ static void mqueue_evict_inode(struct inode *inode)
>>>                 put_ipc_ns(ipc_ns);
>>>  }
>>>
>>> +/*
>>> + * Callback for security_inode_init_security() for acquiring xattrs.
>>> + */
>>> +static int mqueue_initxattrs(struct inode *inode,
>>> +                           const struct xattr *xattr_array,
>>> +                           void *fs_info)
>>> +{
>>> +       struct mqueue_inode_info *info = MQUEUE_I(inode);
>>> +       const struct xattr *xattr;
>>> +       struct simple_xattr *new_xattr;
>>> +       size_t len;
>>> +
>>> +       for (xattr = xattr_array; xattr->name != NULL; xattr++) {
>>> +               new_xattr = simple_xattr_alloc(xattr->value, xattr->value_len);
>>> +               if (!new_xattr)
>>> +                       return -ENOMEM;
>>> +               len = strlen(xattr->name) + 1;
>>> +               new_xattr->name = kmalloc(XATTR_SECURITY_PREFIX_LEN + len,
>>> +                                         GFP_KERNEL);
>>> +               if (!new_xattr->name) {
>>> +                       kfree(new_xattr);
>>> +                       return -ENOMEM;
>>> +               }
>>> +
>>> +               memcpy(new_xattr->name, XATTR_SECURITY_PREFIX,
>>> +                      XATTR_SECURITY_PREFIX_LEN);
>>> +               memcpy(new_xattr->name + XATTR_SECURITY_PREFIX_LEN,
>>> +                      xattr->name, len);
>>> +
>>> +               simple_xattr_list_add(&info->xattrs, new_xattr);
>>> +       }
>>> +
>>> +       return 0;
>>> +}
>>> +
>>>  static int mqueue_create(struct inode *dir, struct dentry *dentry,
>>>                                 umode_t mode, bool excl)
>>>  {
>>> @@ -443,6 +481,14 @@ static int mqueue_create(struct inode *dir, struct dentry *dentry,
>>>                 ipc_ns->mq_queues_count--;
>>>                 goto out_unlock;
>>>         }
>>> +       error = security_inode_init_security(inode, dir,
>>> +                                            &dentry->d_name,
>>> +                                            mqueue_initxattrs, NULL);
>>> +       if (error && error != -EOPNOTSUPP) {
>>> +               spin_lock(&mq_lock);
>>> +               ipc_ns->mq_queues_count--;
>>> +               goto out_unlock;
>>> +       }
>>>
>>>         put_ipc_ns(ipc_ns);
>>>         dir->i_size += DIRENT_SIZE;
>>> --
>>> 1.9.1
>>>
>>> --
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>>
>> --
>> paul moore
>> www.paul-moore.com



-- 
paul moore
www.paul-moore.com