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Date:   Fri, 3 Sep 2021 15:28:22 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH v9 2/3] mm: add a field to store names for private
 anonymous memory
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On Fri, Sep 03, 2021 at 02:56:21PM -0700, Suren Baghdasaryan wrote:
> On Fri, Sep 3, 2021 at 2:47 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > (Sorry, a few more things jumped out at me when I looked again...)
> >
> > On Thu, Sep 02, 2021 at 04:18:12PM -0700, Suren Baghdasaryan wrote:
> > > [...]
> > > diff --git a/kernel/sys.c b/kernel/sys.c
> > > index 72c7639e3c98..25118902a376 100644
> > > --- a/kernel/sys.c
> > > +++ b/kernel/sys.c
> > > @@ -2299,6 +2299,64 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
> > >
> > >  #define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)
> > >
> > > +#ifdef CONFIG_MMU
> > > +
> > > +#define ANON_VMA_NAME_MAX_LEN        256
> > > +
> > > +static inline bool is_valid_name_char(char ch)
> > > +{
> > > +     /* printable ascii characters, except [ \ ] */
> > > +     return (ch > 0x1f && ch < 0x5b) || (ch > 0x5d && ch < 0x7f);
> > > +}
> >
> > In the back of my mind, I feel like disallowing backtick would be nice,
> > but then if $, (, and ) are allowed, it doesn't matter, and that seems
> > too limiting. :)
> 
> It's not used by the only current user (Android) and we can always
> allow more chars later. However going the other direction and
> disallowing some of them I think would be harder (need to make sure
> nobody uses them). WDYT if we keep it stricter and relax if needed?

I'd say, if we can also drop each of: ` $ ( )
then let's do it. Better to keep the obvious shell meta-characters out
of this, although I don't feel strongly about it. Anything that might
get confused by this would be similarly confused by binary names too:

$ cat /proc/3407216/maps
560bdafd4000-560bdafd6000 r--p 00000000 fd:02 2621909 /tmp/yay`wat

And it's probably easier to change a binary name than to call prctl. :P

I'm good either way. What you have now is great, but if we wanted to be
extra extra strict, we can add the other 4 above.

-- 
Kees Cook