From: Casey Schaufler <email@example.com>
To: "Eric W. Biederman" <firstname.lastname@example.org>
Cc: Paul Moore <email@example.com>,
Matthew Wilcox <firstname.lastname@example.org>,
Stephen Smalley <email@example.com>,
Stephen Brennan <firstname.lastname@example.org>,
Alexey Dobriyan <email@example.com>,
James Morris <firstname.lastname@example.org>,
"Serge E. Hallyn" <email@example.com>,
Eric Paris <firstname.lastname@example.org>,
email@example.com, Alexander Viro <firstname.lastname@example.org>,
Casey Schaufler <email@example.com>
Subject: Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU
Date: Tue, 15 Dec 2020 14:53:44 -0800 [thread overview]
Message-ID: <firstname.lastname@example.org> (raw)
On 12/15/2020 2:04 PM, Eric W. Biederman wrote:
> Casey Schaufler <email@example.com> writes:
>> On 12/13/2020 3:00 PM, Paul Moore wrote:
>>> On Sun, Dec 13, 2020 at 11:30 AM Matthew Wilcox <firstname.lastname@example.org> wrote:
>>>> On Sun, Dec 13, 2020 at 08:22:32AM -0600, Eric W. Biederman wrote:
>>>>> Matthew Wilcox <email@example.com> 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);
> isec->sclass = inode_mode_to_security_class(inode->i_mode);
> isec->sid = sid;
> isec->initialized = LABEL_INITIALIZED;
> 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
> 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?
next prev parent reply other threads:[~2020-12-15 22:54 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-04 0:02 [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU Stephen Brennan
2020-12-12 20:55 ` Matthew Wilcox
2020-12-13 14:22 ` Eric W. Biederman
2020-12-13 16:29 ` Matthew Wilcox
2020-12-13 23:00 ` Paul Moore
2020-12-15 18:09 ` Casey Schaufler
2020-12-15 22:04 ` Eric W. Biederman
2020-12-15 22:53 ` Casey Schaufler [this message]
2020-12-16 1:05 ` Stephen Brennan
2020-12-14 18:45 ` Casey Schaufler
2020-12-14 18:15 ` Stephen Brennan
2020-12-13 14:30 ` Eric W. Biederman
2020-12-13 16:32 ` Matthew Wilcox
2020-12-14 17:19 ` Stephen Brennan
2020-12-15 21:45 ` Eric W. Biederman
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