Commit bd56086f authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Linus Torvalds
Browse files

thp: fix split_huge_page() after mremap() of THP 

Sasha Levin has reported KASAN out-of-bounds bug[1].  It points to "if
(!is_swap_pte(pte[i]))" in unfreeze_page_vma() as a problematic access.

The cause is that split_huge_page() doesn't handle THP correctly if it's
not allingned to PMD boundary.  It can happen after mremap().

Test-case (not always triggers the bug):

	#define _GNU_SOURCE
	#include <stdio.h>
	#include <stdlib.h>
	#include <sys/mman.h>

	#define MB (1024UL*1024)
	#define SIZE (2*MB)
	#define BASE ((void *)0x400000000000)

	int main()
	{
		char *p;

		p = mmap(BASE, SIZE, PROT_READ | PROT_WRITE,
				MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE,
				-1, 0);
		if (p == MAP_FAILED)
			perror("mmap"), exit(1);
		p = mremap(BASE, SIZE, SIZE, MREMAP_FIXED | MREMAP_MAYMOVE,
				BASE + SIZE + 8192);
		if (p == MAP_FAILED)
			perror("mremap"), exit(1);
		system("echo 1 > /sys/kernel/debug/split_huge_pages");
		return 0;
	}

The patch fixes freeze and unfreeze paths to handle page table boundary
crossing.

It also makes mapcount vs count check in split_huge_page_to_list()
stricter:
 - after freeze we don't expect any subpage mapped as we remove them
   from rmap when setting up migration entries;
 - count must be 1, meaning only caller has reference to the page;

[1] https://gist.github.com/sashalevin/c67fbea55e7c0576972a



Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: default avatarSasha Levin <sasha.levin@oracle.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent b8d3c4c3

mm/huge_memory.c

+49 −21
Original line number Diff line number Diff line
@@ -2990,6 +2990,7 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma, 
static void freeze_page_vma(struct vm_area_struct *vma, struct page *page,

		unsigned long address)

{

	unsigned long haddr = address & HPAGE_PMD_MASK;

	spinlock_t *ptl;

	pgd_t *pgd;

	pud_t *pud;
@@ -3019,34 +3020,47 @@ static void freeze_page_vma(struct vm_area_struct *vma, struct page *page, 
	}

	if (pmd_trans_huge(*pmd)) {

		if (page == pmd_page(*pmd))

			__split_huge_pmd_locked(vma, pmd, address, true);

			__split_huge_pmd_locked(vma, pmd, haddr, true);

		spin_unlock(ptl);

		return;

	}

	spin_unlock(ptl);



	pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);

	for (i = 0; i < nr; i++, address += PAGE_SIZE, page++) {

	for (i = 0; i < nr; i++, address += PAGE_SIZE, page++, pte++) {

		pte_t entry, swp_pte;

		swp_entry_t swp_entry;



		if (!pte_present(pte[i]))

		/*

		 * We've just crossed page table boundary: need to map next one.

		 * It can happen if THP was mremaped to non PMD-aligned address.

		 */

		if (unlikely(address == haddr + HPAGE_PMD_SIZE)) {

			pte_unmap_unlock(pte - 1, ptl);

			pmd = mm_find_pmd(vma->vm_mm, address);

			if (!pmd)

				return;

			pte = pte_offset_map_lock(vma->vm_mm, pmd,

					address, &ptl);

		}



		if (!pte_present(*pte))

			continue;

		if (page_to_pfn(page) != pte_pfn(pte[i]))

		if (page_to_pfn(page) != pte_pfn(*pte))

			continue;

		flush_cache_page(vma, address, page_to_pfn(page));

		entry = ptep_clear_flush(vma, address, pte + i);

		entry = ptep_clear_flush(vma, address, pte);

		if (pte_dirty(entry))

			SetPageDirty(page);

		swp_entry = make_migration_entry(page, pte_write(entry));

		swp_pte = swp_entry_to_pte(swp_entry);

		if (pte_soft_dirty(entry))

			swp_pte = pte_swp_mksoft_dirty(swp_pte);

		set_pte_at(vma->vm_mm, address, pte + i, swp_pte);

		set_pte_at(vma->vm_mm, address, pte, swp_pte);

		page_remove_rmap(page, false);

		put_page(page);

	}

	pte_unmap_unlock(pte, ptl);

	pte_unmap_unlock(pte - 1, ptl);

}



static void freeze_page(struct anon_vma *anon_vma, struct page *page)
@@ -3058,14 +3072,13 @@ static void freeze_page(struct anon_vma *anon_vma, struct page *page) 


	anon_vma_interval_tree_foreach(avc, &anon_vma->rb_root, pgoff,

			pgoff + HPAGE_PMD_NR - 1) {

		unsigned long haddr;

		unsigned long address = __vma_address(page, avc->vma);



		haddr = __vma_address(page, avc->vma) & HPAGE_PMD_MASK;

		mmu_notifier_invalidate_range_start(avc->vma->vm_mm,

				haddr, haddr + HPAGE_PMD_SIZE);

		freeze_page_vma(avc->vma, page, haddr);

				address, address + HPAGE_PMD_SIZE);

		freeze_page_vma(avc->vma, page, address);

		mmu_notifier_invalidate_range_end(avc->vma->vm_mm,

				haddr, haddr + HPAGE_PMD_SIZE);

				address, address + HPAGE_PMD_SIZE);

	}

}
@@ -3076,6 +3089,7 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page, 
	pmd_t *pmd;

	pte_t *pte, entry;

	swp_entry_t swp_entry;

	unsigned long haddr = address & HPAGE_PMD_MASK;

	int i, nr = HPAGE_PMD_NR;



	/* Skip pages which doesn't belong to the VMA */
@@ -3089,12 +3103,26 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page, 
	pmd = mm_find_pmd(vma->vm_mm, address);

	if (!pmd)

		return;



	pte = pte_offset_map_lock(vma->vm_mm, pmd, address, &ptl);

	for (i = 0; i < nr; i++, address += PAGE_SIZE, page++) {

		if (!is_swap_pte(pte[i]))

	for (i = 0; i < nr; i++, address += PAGE_SIZE, page++, pte++) {

		/*

		 * We've just crossed page table boundary: need to map next one.

		 * It can happen if THP was mremaped to non-PMD aligned address.

		 */

		if (unlikely(address == haddr + HPAGE_PMD_SIZE)) {

			pte_unmap_unlock(pte - 1, ptl);

			pmd = mm_find_pmd(vma->vm_mm, address);

			if (!pmd)

				return;

			pte = pte_offset_map_lock(vma->vm_mm, pmd,

					address, &ptl);

		}



		if (!is_swap_pte(*pte))

			continue;



		swp_entry = pte_to_swp_entry(pte[i]);

		swp_entry = pte_to_swp_entry(*pte);

		if (!is_migration_entry(swp_entry))

			continue;

		if (migration_entry_to_page(swp_entry) != page)
@@ -3110,12 +3138,12 @@ static void unfreeze_page_vma(struct vm_area_struct *vma, struct page *page, 
			entry = maybe_mkwrite(entry, vma);



		flush_dcache_page(page);

		set_pte_at(vma->vm_mm, address, pte + i, entry);

		set_pte_at(vma->vm_mm, address, pte, entry);



		/* No need to invalidate - it was non-present before */

		update_mmu_cache(vma, address, pte + i);

		update_mmu_cache(vma, address, pte);

	}

	pte_unmap_unlock(pte, ptl);

	pte_unmap_unlock(pte - 1, ptl);

}



static void unfreeze_page(struct anon_vma *anon_vma, struct page *page)
@@ -3321,7 +3349,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) 
	spin_lock(&split_queue_lock);

	count = page_count(head);

	mapcount = total_mapcount(head);

	if (mapcount == count - 1) {

	if (!mapcount && count == 1) {

		if (!list_empty(page_deferred_list(head))) {

			split_queue_len--;

			list_del(page_deferred_list(head));
@@ -3329,13 +3357,13 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) 
		spin_unlock(&split_queue_lock);

		__split_huge_page(page, list);

		ret = 0;

	} else if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount > count - 1) {

	} else if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {

		spin_unlock(&split_queue_lock);

		pr_alert("total_mapcount: %u, page_count(): %u\n",

				mapcount, count);

		if (PageTail(page))

			dump_page(head, NULL);

		dump_page(page, "total_mapcount(head) > page_count(head) - 1");

		dump_page(page, "total_mapcount(head) > 0");

		BUG();

	} else {

		spin_unlock(&split_queue_lock);

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