CVE-2022-2590, Dirty COW restricted to shmem

linux kernel version: Linux/x86 6.0.0-rc1 (commit 37887783b3fef877bf34b8992c9199864da4afcb)

Introduction:

this vulnerability allows the attacker to write arbitrary content to read-only shared memory page by satisfying the function can_follow_write_pte, which checks FOLL_FORCE, FOLL_COW, and pte_dirty, using UFFDIO_CONTINUE.

Analysis:

/*
 * FOLL_FORCE can write to even unwritable pte's, but only
 * after we've gone through a COW cycle and they are dirty.
 */
static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
	return pte_write(pte) ||
		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}

The function above determines whether a pte is writable.

The pte is considered writable, if it meets one of the following conditions:

The pte's flags include writable flag.
The flags variable contains the FOLL_FORCE and FOLL_COW flags, and the pte's flags include dirty flag.

To satisfy condition 2, the following three flags need to be set:

FOLL_FORCE

static ssize_t mem_rw(struct file *file, char __user *buf,
			size_t count, loff_t *ppos, int write)
{
	...

	flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);

	while (count > 0) {
		size_t this_len = min_t(size_t, count, PAGE_SIZE);

		if (write && copy_from_user(page, buf, this_len)) {
			copied = -EFAULT;
			break;
		}

		this_len = access_remote_vm(mm, addr, page, this_len, flags); // __get_user_pages with FOLL_FORCE on
		if (!this_len) {
			if (!copied)
				copied = -EIO;
			break;
		}

		...
}

The above function performs read/write operations on /proc/<pid>/mem.

By using this function, you can reach the __get_user_pages function with the FOLL_FORCE flag on.

Although FOLL_FORCE is meant to be used in ptrace, its necessity has been proved in the following commit: https://github.com/torvalds/linux/commit/f511c0b17b081562dca8ac5061dfa86db4c66cc2

FOLL_COW

/*
* mmap_lock must be held on entry.  If @locked != NULL and *@flags
* does not include FOLL_NOWAIT, the mmap_lock may be released.  If it
* is, *@locked will be set to 0 and -EBUSY returned.
*/
static int faultin_page(struct vm_area_struct *vma,
	unsigned long address, unsigned int *flags, bool unshare,
	int *locked)
{
  ...

  /*
  * The VM_FAULT_WRITE bit tells us that do_wp_page has broken COW when
  * necessary, even if maybe_mkwrite decided not to set pte_write. We
  * can thus safely do subsequent page lookups as if they were reads.
  * But only do so when looping for pte_write is futile: in some cases
  * userspace may also be wanting to write to the gotten user page,
  * which a read fault here might prevent (a readonly page might get
  * reCOWed by userspace write).
  */
  if ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE))
    *flags |= FOLL_COW;
  return 0;
}

OR operation with FOLL_COW in the faultin_page function can be used, which is a part of the patch for the Dirty COW.

static long __get_user_pages(struct mm_struct *mm,
 		unsigned long start, unsigned long nr_pages,
 		unsigned int gup_flags, struct page **pages,
 		struct vm_area_struct **vmas, int *locked)
 {
 	...
 retry:
 		/*
 		 * If we have a pending SIGKILL, don't keep faulting pages and
 		 * potentially allocating memory.
 		 */
 		if (fatal_signal_pending(current)) {
 			ret = -EINTR;
 			goto out;
 		}
 		cond_resched();

 		page = follow_page_mask(vma, start, foll_flags, &ctx); // can_follow_write_pte
 		if (!page || PTR_ERR(page) == -EMLINK) {
 			ret = faultin_page(vma, start, &foll_flags,
 					   PTR_ERR(page) == -EMLINK, locked); // flags |= FOLL_COW
 			switch (ret) {
 			case 0:
 				goto retry; // try follow page again
 			case -EBUSY:
 			case -EAGAIN:
 				ret = 0;
 				fallthrough;
 			case -EFAULT:
 			case -ENOMEM:
 			case -EHWPOISON:
 				goto out;
 			}
 			BUG();
 		} else if (PTR_ERR(page) == -EEXIST) {
    ...
 }

In order to reach the can_follow_write_pte function with the FOLL_COW flag on, you need to call the faultin_page function within the __get_user_pages function, redirect to the retry label, and call the faultin_page function again.

pte_dirty

This condition can be satisfied due to the following commit: https://github.com/torvalds/linux/commit/9ae0f87d009ca6c4aab2882641ddfc319727e3db

/*
* Install PTEs, to map dst_addr (within dst_vma) to page.
*
* This function handles both MCOPY_ATOMIC_NORMAL and _CONTINUE for both shmem
* and anon, and for both shared and private VMAs.
*/
int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
		     struct vm_area_struct *dst_vma,
		     unsigned long dst_addr, struct page *page,
		     bool newly_allocated, bool wp_copy)
{
 	...

 	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);
 	_dst_pte = pte_mkdirty(_dst_pte); // set pte dirty unconditionally
 	if (page_in_cache && !vm_shared)
 		writable = false;

 	...
}

Due to the above patch, it is possible to install a pte for the read-only shared memory page in a dirty state unconditionally.

If those three flags are satisfied, the can_follow_write_pte function will return true.

static struct page *follow_page_pte(struct vm_area_struct *vma,
		unsigned long address, pmd_t *pmd, unsigned int flags,
		struct dev_pagemap **pgmap)
{
	...
    
	// true && !true == false
	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
		pte_unmap_unlock(ptep, ptl);
		return NULL;
	}

	page = vm_normal_page(vma, address, pte); // get read-only shared memory page

	...

out:
	pte_unmap_unlock(ptep, ptl);
	return page;
no_page:
	pte_unmap_unlock(ptep, ptl);
	if (!pte_none(pte))
		return NULL;
	return no_page_table(vma, flags);
}

The function follow_page_pte returns the read-only shared memory page.

Race Scenario:

The following is the race scenario proved by the PoC, which will be introduced later.

madvise and read	UFFDIO_CONTINUE ioctl	pwrite
madvise // zap the page
shmem_fault // read fault
handle_userfault
	userfaultfd_continue
	mcontinue_atomic_pte
	ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); // get page
	mfill_atomic_install_pte
	_dst_pte = pte_mkdirty(_dst_pte); // make pte dirty
	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte); // install pte	mem_rw
		access_remote_vm // with FOLL_FORCE
		__get_user_pages
		can_follow_write_pte // no FOLL_COW, return 0
		faultin_page
		flags
		retry:
		follow_page_pte
madvise // zap the page
shmem_fault // read fault
handle_userfault
	userfaultfd_continue
	mcontinue_atomic_pte
	ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); // get page
	mfill_atomic_install_pte
	_dst_pte = pte_mkdirty(_dst_pte); // make pte dirty
	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte); // install pte
		can_follow_write_pte // return 1
		copy_to_user(buf, page, this_len) // write content to read-only page

The madvise and read routines are executed twice; the first execution induces a retry of __get_user_pages, and the second execution guides the follow_page_pte to return the page pointed by pte that has been made dirty by mfill_atomic_install_pte.

PoC and Patch:

https://www.openwall.com/lists/oss-security/2022/08/15/1

You can check David Hildenbrand's reproducer at the link above.

However, since the reproducer makes it difficult to recognize the exact race scenario, I modified the reproducer to prefer a more linear execution, and accordingly, modified the Linux kernel source code as well.

.config:

...
CONFIG_USERFAULTFD=y
CONFIG_HAVE_ARCH_USERFAULTFD_WP=y
CONFIG_HAVE_ARCH_USERFAULTFD_MINOR=y
...

PoC: (poc.c)

Patch: (poc_deayzl.patch)

For testing, the following preliminary steps must be done. (The instructions in david's reproducer says the file path is /tmp/foo. But tmpfs is not shared memory in the current version, so the userfaultfd register fails)

sudo -s
echo "asdf" > /dev/shm/foo
chmod 0404 /dev/shm/foo
exit

After running the PoC, you will see the following screen.

References:

commit: https://github.com/torvalds/linux/commit/5535be3099717646781ce1540cf725965d680e7b

lore.kernel patch v1: https://lore.kernel.org/linux-mm/20220808073232.8808-1-david@redhat.com/#r

lore.kernel patch v2: https://lore.kernel.org/all/20220809205640.70916-1-david@redhat.com/T/#u

openwall: https://www.openwall.com/lists/oss-security/2022/08/08/1