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

Jeff Xu
2023-05-16T22:25:32Z

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