Re: [PATCH 5/6] KEY: Apply PKEY_ENFORCE_API to munmap

[Jeff Xu <jeffxu@google.com>]

On Tue, May 16, 2023 at 1:13 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, May 15, 2023 at 01:05:51PM +0000, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@google.com>
> >
> > This patch enables PKEY_ENFORCE_API for the munmap
> > syscall.
> >
> > Signed-off-by: Jeff Xu<jeffxu@google.com>
> > ---
> >  include/linux/mm.h |  2 +-
> >  mm/mmap.c          | 34 ++++++++++++++++++++++++++--------
> >  mm/mremap.c        |  6 ++++--
> >  3 files changed, 31 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 27ce77080c79..48076e845d53 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3136,7 +3136,7 @@ extern unsigned long do_mmap(struct file *file, unsigned long addr,
> >       unsigned long pgoff, unsigned long *populate, struct list_head *uf);
> >  extern int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
> >                        unsigned long start, size_t len, struct list_head *uf,
> > -                      bool downgrade);
> > +                      bool downgrade, bool syscall);
>
> For type checking and readability, I suggest using an enum instead of
> "bool". Perhaps something like:
>
> enum caller_origin {
>         ON_BEHALF_OF_KERNEL = 0,
>         ON_BEHALF_OF_USERSPACE,
> };
>
Sure, it makes sense.


>  extern int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
>                          unsigned long start, size_t len, struct list_head *uf,
> -                        bool downgrade);
> +                        bool downgrade, enum caller_origin called);
>
> >  extern int do_munmap(struct mm_struct *, unsigned long, size_t,
> >                    struct list_head *uf);
> >  extern int do_madvise(struct mm_struct *mm, unsigned long start, size_t len_in, int behavior);
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 13678edaa22c..29329aa794a6 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2498,6 +2498,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >   * @uf: The userfaultfd list_head
> >   * @downgrade: set to true if the user wants to attempt to write_downgrade the
> >   * mmap_lock
> > + * @syscall: set to true if this is called from syscall entry
> >   *
> >   * This function takes a @mas that is either pointing to the previous VMA or set
> >   * to MA_START and sets it up to remove the mapping(s).  The @len will be
> > @@ -2507,7 +2508,7 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> >   */
> >  int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
> >                 unsigned long start, size_t len, struct list_head *uf,
> > -               bool downgrade)
> > +               bool downgrade, bool syscall)
> >  {
> >       unsigned long end;
> >       struct vm_area_struct *vma;
> > @@ -2519,6 +2520,19 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
> >       if (end == start)
> >               return -EINVAL;
> >
> > +     /*
> > +      * When called by syscall from userspace, check if the calling
> > +      * thread has the PKEY permission to modify the memory mapping.
> > +      */
> > +     if (syscall && arch_check_pkey_enforce_api(mm, start, end) < 0) {
>
>         if (called == ON_BEHALF_OF_USERSPACE &&
>             arch_check_pkey_enforce_api(mm, start, end) < 0) {
>
> > +             char comm[TASK_COMM_LEN];
> > +
> > +             pr_warn_ratelimited(
> > +                     "munmap was denied on PKEY_ENFORCE_API memory, pid=%d '%s'\n",
> > +                     task_pid_nr(current), get_task_comm(comm, current));
> > +             return -EACCES;
> > +     }
> > +
> >        /* arch_unmap() might do unmaps itself.  */
> >       arch_unmap(mm, start, end);
> >
> > @@ -2541,7 +2555,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
> >  {
> >       VMA_ITERATOR(vmi, mm, start);
> >
> > -     return do_vmi_munmap(&vmi, mm, start, len, uf, false);
> > +     return do_vmi_munmap(&vmi, mm, start, len, uf, false, false);
>
> +       return do_vmi_munmap(&vmi, mm, start, len, uf, false, ON_BEHALF_OF_KERNEL);
>
> > [...]
> >  SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len)
> >  {
> >       addr = untagged_addr(addr);
> > -     return __vm_munmap(addr, len, true);
> > +     return __vm_munmap(addr, len, true, true);
>
> +       return __vm_munmap(addr, len, true, ON_BEHALF_OF_USERSPACE);
>
> etc.
>
> --
> Kees Cook

Thanks!
-Jeff Xu