[PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

[Casey Schaufler]

On 12/15/2020 2:04 PM, Eric W. Biederman wrote:
> Casey Schaufler <casey at schaufler-ca.com> writes:
>
>> On 12/13/2020 3:00 PM, Paul Moore wrote:
>>> On Sun, Dec 13, 2020 at 11:30 AM Matthew Wilcox <willy at infradead.org> wrote:
>>>> On Sun, Dec 13, 2020 at 08:22:32AM -0600, Eric W. Biederman wrote:
>>>>> Matthew Wilcox <willy at infradead.org> writes:
>>>>>
>>>>>> On Thu, Dec 03, 2020 at 04:02:12PM -0800, Stephen Brennan wrote:
>>>>>>> -void pid_update_inode(struct task_struct *task, struct inode *inode)
>>>>>>> +static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
>>>>>>> +                         unsigned int flags)
>>>>>> I'm really nitpicking here, but this function only _updates_ the inode
>>>>>> if flags says it should.  So I was thinking something like this
>>>>>> (compile tested only).
>>>>>>
>>>>>> I'd really appreocate feedback from someone like Casey or Stephen on
>>>>>> what they need for their security modules.
>>>>> Just so we don't have security module questions confusing things
>>>>> can we please make this a 2 patch series?  With the first
>>>>> patch removing security_task_to_inode?
>>>>>
>>>>> The justification for the removal is that all security_task_to_inode
>>>>> appears to care about is the file type bits in inode->i_mode.  Something
>>>>> that never changes.  Having this in a separate patch would make that
>>>>> logical change easier to verify.
>>>> I don't think that's right, which is why I keep asking Stephen & Casey
>>>> for their thoughts.
>>> The SELinux security_task_to_inode() implementation only cares about
>>> inode->i_mode S_IFMT bits from the inode so that we can set the object
>>> class correctly.  The inode's SELinux label is taken from the
>>> associated task.
>>>
>>> Casey would need to comment on Smack's needs.
>> SELinux uses different "class"es on subjects and objects.
>> Smack does not differentiate, so knows the label it wants
>> the inode to have when smack_task_to_inode() is called,
>> and sets it accordingly. Nothing is allocated in the process,
>> and the new value is coming from the Smack master label list.
>> It isn't going to go away. It appears that this is the point
>> of the hook. Am I missing something?
> security_task_to_inode (strangely named as this is proc specific) is
> currently called both when the inode is initialized in proc and when
> pid_revalidate is called and the uid and gid of the proc inode
> are updated to match the traced task.
>
> I am suggesting that the call of security_task_to_inode in
> pid_revalidate be removed as neither of the two implementations of this
> security hook smack nor selinux care of the uid or gid changes.

If you're sure that the only case where pid_revalidate() would matter
is for the uid/gid cases that would be OK.

>
> Removal of the security check will allow proc to be accessed in rcu look
> mode.  AKA give proc go faster stripes.
>
> The two implementations are:
>
> static void selinux_task_to_inode(struct task_struct *p,
> 				  struct inode *inode)
> {
> 	struct inode_security_struct *isec = selinux_inode(inode);
> 	u32 sid = task_sid(p);
>
> 	spin_lock(&isec->lock);
> 	isec->sclass = inode_mode_to_security_class(inode->i_mode);
> 	isec->sid = sid;
> 	isec->initialized = LABEL_INITIALIZED;
> 	spin_unlock(&isec->lock);
> }
>
>
> static void smack_task_to_inode(struct task_struct *p, struct inode *inode)
> {
> 	struct inode_smack *isp = smack_inode(inode);
> 	struct smack_known *skp = smk_of_task_struct(p);
>
> 	isp->smk_inode = skp;
> 	isp->smk_flags |= SMK_INODE_INSTANT;
> }
>
> I see two questions gating the safe removal of the call of
> security_task_to_inode from pid_revalidate.
>
> 1) Does any of this code care about uids or gids.
>    It appears the answer is no from a quick inspection of the code.

It looks that way.

>
> 2) Does smack_task_to_inode need to be called after exec?
>    - Exec especially suid exec changes the the cred on a task.
>    - Execing of a non-leader thread changes the thread_pid of a task
>      so that it is the pid of the entire thread group.

I think so. If SMACK64EXEC is set on a binary the label will
be changed on exec. The /proc inode Smack label would need to
be changed.

>
>    If either of those are significant perhaps we can limit calling
>    security_task_to_inode if task->self_exec_id is different.
>
>    I haven't yet take the time to trace through and see if
>    task_sid(p) or smk_of_task_struct(p) could change based on
>    the security hooks called during exec.  Or how bad the races are if
>    such a change can happen.
>
> Does that clarify the question that is being asked?
>
> Eric