Re: [PATCH v9 2/3] mm: add a field to store names for private anonymous memory

[Suren Baghdasaryan <surenb@google.com>]

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?

>
> > +
> > +static int prctl_set_vma(unsigned long opt, unsigned long addr,
> > +                      unsigned long size, unsigned long arg)
> > +{
> > +     struct mm_struct *mm = current->mm;
> > +     const char __user *uname;
> > +     char *name, *pch;
> > +     int error;
> > +
> > +     switch (opt) {
> > +     case PR_SET_VMA_ANON_NAME:
> > +             uname = (const char __user *)arg;
> > +             if (!uname) {
> > +                     /* Reset the name */
> > +                     name = NULL;
> > +                     goto set_name;
> > +             }
> > +
> > +             name = strndup_user(uname, ANON_VMA_NAME_MAX_LEN);
> > +
> > +             if (IS_ERR(name))
> > +                     return PTR_ERR(name);
> > +
> > +             for (pch = name; *pch != '\0'; pch++) {
> > +                     if (!is_valid_name_char(*pch)) {
> > +                             kfree(name);
> > +                             return -EINVAL;
> > +                     }
> > +             }
> > +set_name:
> > +             mmap_write_lock(mm);
> > +             error = madvise_set_anon_name(mm, addr, size, name);
> > +             mmap_write_unlock(mm);
> > +             kfree(name);
> > +             break;
>
> This is a weird construct with a needless goto. Why not:
>
>         switch (opt) {
>         case PR_SET_VMA_ANON_NAME:
>                 uname = (const char __user *)arg;
>                 if (uname) {
>                         name = strndup_user(uname, ANON_VMA_NAME_MAX_LEN);
>                         if (IS_ERR(name))
>                                 return PTR_ERR(name);
>
>                         for (pch = name; *pch != '\0'; pch++) {
>                                 if (!is_valid_name_char(*pch)) {
>                                         kfree(name);
>                                         return -EINVAL;
>                                 }
>                         }
>                 } else {
>                         /* Reset the name */
>                         name = NULL;
>                 }
>                 mmap_write_lock(mm);
>                 error = madvise_set_anon_name(mm, addr, size, name);
>                 mmap_write_unlock(mm);
>                 kfree(name);
>                 break;

Yeah, I was contemplating one way or the other (less indents vs clear
flow) and you convinced me :) Will change in the next rev.
Thanks for the review!

>
>
> --
> Kees Cook