Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

[Matthew Wilcox]

On Mon, Aug 30, 2021 at 09:16:14AM -0700, Suren Baghdasaryan wrote:
> On Mon, Aug 30, 2021 at 1:12 AM Rasmus Villemoes
> <linux@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > On 28/08/2021 23.47, Suren Baghdasaryan wrote:
> > > On Fri, Aug 27, 2021 at 10:52 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> > >>
> > >>>> +   case PR_SET_VMA_ANON_NAME:
> > >>>> +           name = strndup_user((const char __user *)arg,
> > >>>> +                               ANON_VMA_NAME_MAX_LEN);
> > >>>> +
> > >>>> +           if (IS_ERR(name))
> > >>>> +                   return PTR_ERR(name);
> > >>>> +
> > >>>> +           for (pch = name; *pch != '\0'; pch++) {
> > >>>> +                   if (!isprint(*pch)) {
> > >>>> +                           kfree(name);
> > >>>> +                           return -EINVAL;
> > >>>
> > >>> I think isprint() is too weak a check.  For example, I would suggest
> > >>> forbidding the following characters: ':', ']', '[', ' '.  Perhaps
> >
> > Indeed. There's also the issue that the kernel's ctype actually
> > implements some almost-but-not-quite latin1, so (some) chars above 0x7f
> > would also pass isprint() - while everybody today expects utf-8, so the
> > ability to put almost arbitrary sequences of chars with the high bit set
> > could certainly confuse some parsers. IOW, don't use isprint() at all,
> > just explicitly check for the byte values that we and up agreeing to
> > allow/forbid.
> >
> > >>> isalnum() would be better?  (permit a-zA-Z0-9)  I wouldn't necessarily
> > >>> be opposed to some punctuation characters, but let's avoid creating
> > >>> confusion.  Do you happen to know which characters are actually in use
> > >>> today?
> > >>
> > >> There's some sense in refusing [, ], and :, but removing " " seems
> > >> unhelpful for reasonable descriptors. As long as weird stuff is escaped,
> > >> I think it's fine. Any parser can just extract with m|\[anon:(.*)\]$|
> > >
> > > I see no issue in forbidding '[' and ']' but whitespace and ':' are
> > > currently used by Android. Would forbidding or escaping '[' and ']' be
> > > enough?
> >
> > how about allowing [0x20, 0x7e] except [0x5b, 0x5d], i.e. all printable
> > (including space) ascii characters, except [ \ ] - the brackets as
> > already discussed, and backslash because then there's nobody who can get
> > confused about whether there's some (and then which?) escaping mechanism
> > in play - "\n" is simply never going to appear. Simple rules, easy to
> > implement, easy to explain in a man page.
> 
> Thanks for the suggestion, Rasmus. I'm all for keeping it simple.
> Kees, Matthew, would that be acceptable?

Yes, I think so.  It permits all kinds of characters that might
be confusing if passed on to something else, but we can't prohibit
everything, and forbidding just these three should remove any confusion
for any parser of /proc.  Little Bobby Tables thanks you.