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

[Suren Baghdasaryan]

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?

>
> >>
> >> For example, just escape it here instead of refusing to take it. Something
> >> like:
> >>
> >>         name = strndup_user((const char __user *)arg,
> >>                             ANON_VMA_NAME_MAX_LEN);
> >>         escaped = kasprintf(GFP_KERNEL, "%pE", name);
>
> I would not go down that road. First, it makes it much harder to explain
> the rules for what are allowed and not allowed. Second, parsers become
> much more complicated. Third, does the length limit then apply to the
> escaped or unescaped string?
>
> Rasmus