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

[Stephen Brennan]

Casey Schaufler <casey@xxxxxxxxxxxxxxxx> writes:

> On 12/15/2020 2:04 PM, Eric W. Biederman wrote:
>> Casey Schaufler <casey@xxxxxxxxxxxxxxxx> writes:
>>
>>> On 12/13/2020 3:00 PM, Paul Moore wrote:
>>>> On Sun, Dec 13, 2020 at 11:30 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>>>>> On Sun, Dec 13, 2020 at 08:22:32AM -0600, Eric W. Biederman wrote:
>>>>>> Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

Given these answers then, it seems like a proper implementation would
leave the security_task_to_inode() call in pid_update_inode(). Then,
pid_revalidate() would drop out of RCU mode whenever some function like
this (drawing on Matthew's idea above) returns true:

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 449204e9f749..02805076c42b 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1820,6 +1820,26 @@ void pid_update_inode(struct task_struct *task, struct inode *inode)
 	inode->i_mode &= ~(S_ISUID | S_ISGID);
 }
 
+/* See if we can avoid the above call. Assumes RCU lock held */
+static bool pid_inode_needs_update(struct task_struct *task, struct inode *inode)
+{
+	kuid_t uid;
+	kgid_t gid;
+	u32 exec_id, last_exec_id;
+
+	if (inode->i_mode & (S_ISUID | S_ISGID))
+		return true;
+	task_dump_owner(task, inode->i_mode, &uid, &gid);
+	if (!uid_eq(uid, inode->i_uid) || !gid_eq(gid, inode->i_gid))
+		return true;
+
+	last_exec_id = /* find this stored somewhere? */;
+	task_lock(task);
+	exec_id = task->self_exec_id;
+	task_unlock(task);
+	return exec_id != last_exec_id;
+}
+
 /*
  * Rewrite the inode's ownerships here because the owning task may have
  * performed a setuid(), etc.


Does this make sense?

Stephen

>>
>>    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