Re: [PATCH] prctl: remove one-shot limitation for changing exe link

[Stanislav Kinsburskiy]

12.07.2016 18:42, Oleg Nesterov ÐÐÑÐÑ:
> On 07/12, Stanislav Kinsburskiy wrote:
> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -1696,16 +1696,6 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
> >   		fput(exe_file);
> >   	}
> >   
> > -	/*
> > -	 * The symlink can be changed only once, just to disallow arbitrary
> > -	 * transitions malicious software might bring in. This means one
> > -	 * could make a snapshot over all processes running and monitor
> > -	 * /proc/pid/exe changes to notice unusual activity if needed.
> > -	 */
> > -	err = -EPERM;
> > -	if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags))
> > -		goto exit;
> > -
> I didn't even try to read the changelog so I do not know why do you
> want this change ;)
>
> But I would like to ack it in any case. I never understood why do we
> want/need this MMF_EXE_FILE_CHANGED check, I suggested to remove it
> many times.
>
> And can't resist, please note the xchg() below. Currently (before this
> patch) we do not need it. I was specially added to ensure that we can
> just remove this test_and_set_bit(MMF_EXE_FILE_CHANGED) without adding
> a race.

Thanks, Oleg. I'll take a look.
But should this be addressed in this patch? Especially if it's not 
needed even now (before this patch)?


> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>