Re: [RFC PATCH v2 5/9] fs: add anon_inode_getfile_secure() similar to anon_inode_getfd_secure()

["Mickaël Salaün" <mic@digikod.net>]

On 12/08/2021 16:32, Paul Moore wrote:
> On Thu, Aug 12, 2021 at 5:32 AM Mickaël Salaün <mic@digikod.net> wrote:
>> On 11/08/2021 22:48, Paul Moore wrote:
>>> Extending the secure anonymous inode support to other subsystems
>>> requires that we have a secure anon_inode_getfile() variant in
>>> addition to the existing secure anon_inode_getfd() variant.
>>>
>>> Thankfully we can reuse the existing __anon_inode_getfile() function
>>> and just wrap it with the proper arguments.
>>>
>>> Signed-off-by: Paul Moore <paul@paul-moore.com>
>>>
>>> ---
>>> v2:
>>> - no change
>>> v1:
>>> - initial draft
>>> ---
>>>  fs/anon_inodes.c            |   29 +++++++++++++++++++++++++++++
>>>  include/linux/anon_inodes.h |    4 ++++
>>>  2 files changed, 33 insertions(+)
>>>
>>> diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
>>> index a280156138ed..e0c3e33c4177 100644
>>> --- a/fs/anon_inodes.c
>>> +++ b/fs/anon_inodes.c
>>> @@ -148,6 +148,35 @@ struct file *anon_inode_getfile(const char *name,
>>>  }
>>>  EXPORT_SYMBOL_GPL(anon_inode_getfile);
>>>
>>> +/**
>>> + * anon_inode_getfile_secure - Like anon_inode_getfile(), but creates a new
>>> + *                             !S_PRIVATE anon inode rather than reuse the
>>> + *                             singleton anon inode and calls the
>>> + *                             inode_init_security_anon() LSM hook.  This
>>> + *                             allows for both the inode to have its own
>>> + *                             security context and for the LSM to enforce
>>> + *                             policy on the inode's creation.
>>> + *
>>> + * @name:    [in]    name of the "class" of the new file
>>> + * @fops:    [in]    file operations for the new file
>>> + * @priv:    [in]    private data for the new file (will be file's private_data)
>>> + * @flags:   [in]    flags
>>> + * @context_inode:
>>> + *           [in]    the logical relationship with the new inode (optional)
>>> + *
>>> + * The LSM may use @context_inode in inode_init_security_anon(), but a
>>> + * reference to it is not held.  Returns the newly created file* or an error
>>> + * pointer.  See the anon_inode_getfile() documentation for more information.
>>> + */
>>> +struct file *anon_inode_getfile_secure(const char *name,
>>> +                                    const struct file_operations *fops,
>>> +                                    void *priv, int flags,
>>> +                                    const struct inode *context_inode)
>>> +{
>>> +     return __anon_inode_getfile(name, fops, priv, flags,
>>> +                                 context_inode, true);
>>
>> This is not directly related to this patch but why using the "secure"
>> boolean in __anon_inode_getfile() and __anon_inode_getfd() instead of
>> checking that context_inode is not NULL? This would simplify the code,
>> remove this anon_inode_getfile_secure() wrapper and avoid potential
>> inconsistencies.
> 
> The issue is that it is acceptable for the context_inode to be either
> valid or NULL for callers who request the "secure" code path.
> 
> Look at the SELinux implementation of the anonymous inode hook in
> selinux_inode_init_security_anon() and you will see that in cases
> where the context_inode is valid we simply inherit the label from the
> given inode, whereas if context_inode is NULL we do a type transition
> using the requesting task and the anonymous inode's "name".
> 

Indeed.

Acked-by: Mickaël Salaün <mic@linux.microsoft.com>