Re: [PATCH v7 00/11] extend task comm from 16 to 24

[Yafang Shao]

On Mon, Nov 1, 2021 at 10:07 PM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Mon 2021-11-01 06:04:08, Yafang Shao wrote:
> > There're many truncated kthreads in the kernel, which may make trouble
> > for the user, for example, the user can't get detailed device
> > information from the task comm.
> >
> > This patchset tries to improve this problem fundamentally by extending
> > the task comm size from 16 to 24, which is a very simple way.
> >
> > In order to do that, we have to do some cleanups first.
> >
> > 1. Make the copy of task comm always safe no matter what the task
> >    comm size is. For example,
> >
> >       Unsafe                 Safe
> >       strlcpy                strscpy_pad
> >       strncpy                strscpy_pad
> >       bpf_probe_read_kernel  bpf_probe_read_kernel_str
> >                              bpf_core_read_str
> >                              bpf_get_current_comm
> >                              perf_event__prepare_comm
> >                              prctl(2)
> >
> >    After this step, the comm size change won't make any trouble to the
> >    kernel or the in-tree tools for example perf, BPF programs.
> >
> > 2. Cleanup some old hard-coded 16
> >    Actually we don't need to convert all of them to TASK_COMM_LEN or
> >    TASK_COMM_LEN_16, what we really care about is if the convert can
> >    make the code more reasonable or easier to understand. For
> >    example, some in-tree tools read the comm from sched:sched_switch
> >    tracepoint, as it is derived from the kernel, we'd better make them
> >    consistent with the kernel.
>
> The above changes make sense even if we do not extend comm[] array in
> task_struct.
>
>
> > 3. Extend the task comm size from 16 to 24
> >    task_struct is growing rather regularly by 8 bytes. This size change
> >    should be acceptable. We used to think about extending the size for
> >    CONFIG_BASE_FULL only, but that would be a burden for maintenance
> >    and introduce code complexity.
> >
> > 4. Print a warning if the kthread comm is still truncated.
> >
> > 5. What will happen to the out-of-tree tools after this change?
> >    If the tool get task comm through kernel API, for example prctl(2),
> >    bpf_get_current_comm() and etc, then it doesn't matter how large the
> >    user buffer is, because it will always get a string with a nul
> >    terminator. While if it gets the task comm through direct string copy,
> >    the user tool must make sure the copied string has a nul terminator
> >    itself. As TASK_COMM_LEN is not exposed to userspace, there's no
> >    reason that it must require a fixed-size task comm.
>
> The amount of code that has to be updated is really high. I am pretty
> sure that there are more potential buffer overflows left.
>
> You did not commented on the concerns in the thread
> https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@xxxxxxxxxxxxxx/
>

I thought Steven[1] and  Kees[2] have already clearly explained why we
do it like that, so I didn't give any more words on it.

[1]. https://lore.kernel.org/all/20211025170503.59830a43@xxxxxxxxxxxxxxxxxx/
[2]. https://lore.kernel.org/all/202110251406.56F87A3522@keescook/

> Several people suggested to use a more conservative approach.

Yes, they are Al[3] and Alexei[4].

[3]. https://lore.kernel.org/lkml/YVkmaSUxbg%2FJtBHb@xxxxxxxxxxxxxxxxxxxxx/
[4]. https://lore.kernel.org/all/CAADnVQKm0Ljj-w5PbkAu1ugLFnZRRPt-Vk-J7AhXxDD5xVompA@xxxxxxxxxxxxxx/

> I mean
> to keep comm[16] as is and add a new pointer to the full name. The buffer
> for the long name might be dynamically allocated only when needed.
>

That would add a new allocation in the fork() for the threads with a long name.
I'm not sure if it is worth it.

> The pointer might be either in task_struct or struct kthread. It might
> be used the same way as the full name stored by workqueue kthreads.
>

If we decide to do it like that, I think we'd better add it in
task_struct, then it will work for all tasks.

> The advantage of the separate pointer:
>
>    + would work for names longer than 32
>    + will not open security holes in code
>

Yes, those are the advantages.  And the disadvantage of it is:

 - new allocation in fork()


-- 
Thanks
Yafang