Re: [PATCH] ocfs2: use get_task_comm

[Arnd Bergmann]

On Tue, Dec 5, 2017 at 9:32 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, Dec 5, 2017 at 12:27 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>
>> More generally speaking though, how exactly do we guarantee that
>> there is NUL-termination on tsk->comm during a concurrent update?
>> Could we ever get into a situation where overwrite the NUL byte
>> while setting tsk->comm to a longer string, and read the new start
>> of the string together with an unterminated end, or do we strictly
>> guarantee that the last byte is still NUL? I assume the latter is
>> true, just haven't found exactly where that guarantee is made.
>
> strncpy will zero pad with the trailing NULL, so it's supposed to
> always be safe... still gives me the creeps, though.

But set_task_comm uses strlcpy(), not strncpy(), so you might
get some of the old data back, the question is just whether it could
leak uninitialized data or part of the task_struct up to the next
NUL byte. I could not come up with any code path that would leave
a non-NUL byte in at the end of task->comm though, so it's
probably still safe.

       Arnd