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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 31 Aug 2021 10:21:38 -0700
Message-ID: <CAJuCfpEnZo8nji3R=6ek=2T2TSG=Ke22PJzV_WkYFeSV4R9beg@mail.gmail.com>
Subject: Re: [PATCH v8 2/3] mm: add a field to store names for private
 anonymous memory
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Colin Cross <ccross@google.com>,
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On Mon, Aug 30, 2021 at 9:59 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> 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@rasmusvillemoes.dk> wrote:
> > >
> > > On 28/08/2021 23.47, Suren Baghdasaryan wrote:
> > > > On Fri, Aug 27, 2021 at 10:52 PM Kees Cook <keescook@chromium.org> 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.

Thanks for all the feedback! I think I have enough change suggestions
to resping the next revision. Will send an update later today.