linux-kernel - Re: [PATCH] procfs: fixes pthread cross-thread naming if !PR

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Date:	Tue, 3 May 2016 10:42:18 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Janis Danisevskis <jdanis@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Colin Ian King <colin.king@...onical.com>,
	David Rientjes <rientjes@...gle.com>,
	Minfei Huang <mnfhuang@...il.com>,
	John Stultz <john.stultz@...aro.org>,
	Calvin Owens <calvinowens@...com>, Jann Horn <jann@...jh.net>
Subject: Re: [PATCH] procfs: fixes pthread cross-thread naming if !PR_DUMPABLE

On Tue, May 3, 2016 at 10:25 AM, Janis Danisevskis <jdanis@...gle.com> wrote:
>
>
> On 26/04/16 21:14, Kees Cook wrote:
>>
>> On Tue, Apr 26, 2016 at 10:20 AM, Janis Danisevskis <jdanis@...gle.com>
>> wrote:
>>>
>>> The PR_DUMPABLE flag causes the pid related paths of the
>>> proc file system to be owned by ROOT. The implementation
>>> of pthread_set/getname_np however needs access to
>>> /proc/<pid>/task/<tid>/comm.
>>> If PR_DUMPABLE is false this implementation is locked out.
>>>
>>> This patch installs a special permission function for
>>> the file "comm" that grants read and write access to
>>> all threads of the same group regardless of the ownership
>>> of the inode. For all other threads the function falls back
>>> to the generic inode permission check.
>>>
>>> Signed-off-by: Janis Danisevskis <jdanis@...gle.com>
>>
>>
>> Instead of a permissions function, perhaps this should be handled in
>> the open() of proc_pid_set_comm_operations (and the REG permissions
>> loosened)? I'm concerned there's a race here between the perm check
>> and the resulting open. I'd rather have the open doing the check to
>> eliminate the race.
>
>
> I kind of thought that the permission check is on the open path
> could you elaborate on the race that you are expecting?

In looking I see now that comm_write() still retains its
same_thread_group() check, so nevermind about the race. I was thinking
it was gone, so that the pid could change between the permissions
check and the write.

> Also, in what way would you loosen the permissions on the REG?
> If the DUMPABLE flag is cleared this node is owned by ROOT.
> So the only way to make it writable to a user process would be
> to make it world writable. This cannot be your intention.

I meant to do all the access control in the open() routine to make the
world-writable permissions irrelevant. But, I think, your solution is
easier to read. :)

One thing I can't find, though, is where PR_SET_DUMPABLE makes these
uid changes. I only see uid changes happening when the cred changes
(which then triggers the dumpable change). What's the process flow
that gets a thread into this state?

-Kees

>
> Janis
>
>
>>
>> -Kees
>>
>>> ---
>>>   fs/proc/base.c | 42 +++++++++++++++++++++++++++++++++++++++++-
>>>   1 file changed, 41 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>> index b1755b2..c8ceb3c8 100644
>>> --- a/fs/proc/base.c
>>> +++ b/fs/proc/base.c
>>> @@ -3157,6 +3157,44 @@ int proc_pid_readdir(struct file *file, struct
>>> dir_context *ctx)
>>>   }
>>>
>>>   /*
>>> + * proc_tid_comm_permission is a special permission function exclusively
>>> + * used for the node /proc/<pid>/task/<tid>/comm.
>>> + * It bypasses generic permission checks in the case where a task of the
>>> same
>>> + * task group attempts to access the node.
>>> + * The rational behind this is that glibc and bionic access this node
>>> for
>>> + * cross thread naming (pthread_set/getname_np(!self)). However, if
>>> + * PR_SET_DUMPABLE gets set to 0 this node among others becomes uid=0
>>> gid=0,
>>> + * which locks out the cross thread naming implementation.
>>> + * This function makes sure that the node is always accessible for
>>> members of
>>> + * same thread group.
>>> + */
>>> +static int proc_tid_comm_permission(struct inode *inode, int mask)
>>> +{
>>> +       bool is_same_tgroup;
>>> +       struct task_struct *task;
>>> +
>>> +       task = get_proc_task(inode);
>>> +       if (!task)
>>> +               return -ESRCH;
>>> +       is_same_tgroup = same_thread_group(current, task);
>>> +       put_task_struct(task);
>>> +
>>> +       if (likely(is_same_tgroup && !(mask & MAY_EXEC))) {
>>> +               /* This file (/proc/<pid>/task/<tid>/comm) can always be
>>> +                * read or written by the members of the corresponding
>>> +                * thread group.
>>> +                */
>>> +               return 0;
>>> +       }
>>> +
>>> +       return generic_permission(inode, mask);
>>> +}
>>> +
>>> +static const struct inode_operations proc_tid_comm_inode_operations = {
>>> +               .permission = proc_tid_comm_permission,
>>> +};
>>> +
>>> +/*
>>>    * Tasks
>>>    */
>>>   static const struct pid_entry tid_base_stuff[] = {
>>> @@ -3174,7 +3212,9 @@ static const struct pid_entry tid_base_stuff[] = {
>>>   #ifdef CONFIG_SCHED_DEBUG
>>>          REG("sched",     S_IRUGO|S_IWUSR, proc_pid_sched_operations),
>>>   #endif
>>> -       REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
>>> +       NOD("comm",      S_IFREG|S_IRUGO|S_IWUSR,
>>> +                        &proc_tid_comm_inode_operations,
>>> +                        &proc_pid_set_comm_operations, {}),
>>>   #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
>>>          ONE("syscall",   S_IRUSR, proc_pid_syscall),
>>>   #endif
>>> --
>>> 2.8.0.rc3.226.g39d4020
>>>
>>
>>
>>
>



-- 
Kees Cook
Chrome OS & Brillo Security