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

Kees Cook
2023-05-16T20:07:05Z

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,
};

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