Re: [PATCH] procfs: fixes pthread cross-thread naming if !PR_DUMPABLE

[Janis Danisevskis <jdanis@google.com>]

On 03/05/16 18:42, Kees Cook wrote:
> On Tue, May 3, 2016 at 10:25 AM, Janis Danisevskis <jdanis@google.com> wrote:
>>
>>
>> On 26/04/16 21:14, Kees Cook wrote:
>>>
>>> On Tue, Apr 26, 2016 at 10:20 AM, Janis Danisevskis <jdanis@google.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@google.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?

In fs/proc/base.c look for task_dumpable. It happens in the revalidate
functions and also when the nodes are first instantiated.

Janis

>
> -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
>>>>
>>>
>>>
>>>
>>
>
>
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