Re: [PATCH 4/7] fs/binfmt_elf: use get_task_comm instead of open-coded string copy

[Yafang Shao]

On Thu, Nov 11, 2021 at 7:35 PM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Thu 2021-11-11 11:06:04, David Hildenbrand wrote:
> > On 11.11.21 11:03, David Hildenbrand wrote:
> > > On 08.11.21 09:38, Yafang Shao wrote:
> > >> It is better to use get_task_comm() instead of the open coded string
> > >> copy as we do in other places.
> > >>
> > >> struct elf_prpsinfo is used to dump the task information in userspace
> > >> coredump or kernel vmcore. Below is the verfication of vmcore,
> > >>
> > >> crash> ps
> > >>    PID    PPID  CPU       TASK        ST  %MEM     VSZ    RSS  COMM
> > >>       0      0   0  ffffffff9d21a940  RU   0.0       0      0  [swapper/0]
> > >>>     0      0   1  ffffa09e40f85e80  RU   0.0       0      0  [swapper/1]
> > >>>     0      0   2  ffffa09e40f81f80  RU   0.0       0      0  [swapper/2]
> > >>>     0      0   3  ffffa09e40f83f00  RU   0.0       0      0  [swapper/3]
> > >>>     0      0   4  ffffa09e40f80000  RU   0.0       0      0  [swapper/4]
> > >>>     0      0   5  ffffa09e40f89f80  RU   0.0       0      0  [swapper/5]
> > >>       0      0   6  ffffa09e40f8bf00  RU   0.0       0      0  [swapper/6]
> > >>>     0      0   7  ffffa09e40f88000  RU   0.0       0      0  [swapper/7]
> > >>>     0      0   8  ffffa09e40f8de80  RU   0.0       0      0  [swapper/8]
> > >>>     0      0   9  ffffa09e40f95e80  RU   0.0       0      0  [swapper/9]
> > >>>     0      0  10  ffffa09e40f91f80  RU   0.0       0      0  [swapper/10]
> > >>>     0      0  11  ffffa09e40f93f00  RU   0.0       0      0  [swapper/11]
> > >>>     0      0  12  ffffa09e40f90000  RU   0.0       0      0  [swapper/12]
> > >>>     0      0  13  ffffa09e40f9bf00  RU   0.0       0      0  [swapper/13]
> > >>>     0      0  14  ffffa09e40f98000  RU   0.0       0      0  [swapper/14]
> > >>>     0      0  15  ffffa09e40f9de80  RU   0.0       0      0  [swapper/15]
> > >>
> > >> It works well as expected.
> > >>
> > >> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > >> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > >> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > >> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> > >> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> > >> Cc: Michal Miroslaw <mirq-linux@xxxxxxxxxxxx>
> > >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > >> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> > >> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > >> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > >> Cc: Petr Mladek <pmladek@xxxxxxxx>
> > >> ---
> > >>  fs/binfmt_elf.c | 2 +-
> > >>  1 file changed, 1 insertion(+), 1 deletion(-)
> > >>
> > >> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > >> index a813b70f594e..138956fd4a88 100644
> > >> --- a/fs/binfmt_elf.c
> > >> +++ b/fs/binfmt_elf.c
> > >> @@ -1572,7 +1572,7 @@ static int fill_psinfo(struct elf_prpsinfo *psinfo, struct task_struct *p,
> > >>    SET_UID(psinfo->pr_uid, from_kuid_munged(cred->user_ns, cred->uid));
> > >>    SET_GID(psinfo->pr_gid, from_kgid_munged(cred->user_ns, cred->gid));
> > >>    rcu_read_unlock();
> > >> -  strncpy(psinfo->pr_fname, p->comm, sizeof(psinfo->pr_fname));
> > >> +  get_task_comm(psinfo->pr_fname, p);
> > >>
> > >>    return 0;
> > >>  }
> > >>
> > >
> > > We have a hard-coded "pr_fname[16]" as well, not sure if we want to
> > > adjust that to use TASK_COMM_LEN?
> >
> > But if the intention is to chance TASK_COMM_LEN later, we might want to
> > keep that unchanged.
>
> It seems that len will not change in the end. Another solution is
> going to be used for the long names, see
> https://lore.kernel.org/r/20211108084142.4692-1-laoar.shao@xxxxxxxxx.
>
> > (replacing the 16 by a define might still be a good idea, similar to how
> > it's done for ELF_PRARGSZ, but just a thought)
>
> If the code would need some tweaking when the size changes, you could
> still use TASK_COMM_LEN and trigger a compilation error when the size
> gets modified. For example, static_assert(TASK_COMM_LEN == 16);
>
> It will make it clear that it needs attention if the size is ever modified.
>

I think we can just add some comments to make it grepable, for example,

+  /* TASK_COMM_LEN */
    char pr_fname[16];

or a more detailed explanation:

+  /*
+   * The hard-coded 16 is derived from TASK_COMM_LEN, but it can be changed as
+   *  it is exposed to userspace. We'd better make it hard-coded here.
+   */
char pr_fname[16];

-- 
Thanks
Yafang