Commit d8fd84dd authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

mm/hugetlb: don't map folios writable without VM_WRITE when copying during fork() 

If we have to trigger a hugetlb folio copy during fork() because the anon
folio might be pinned, we currently unconditionally create a writable PTE.

However, the VMA might not have write permissions (VM_WRITE) at that
point.

Fix it by checking the VMA for VM_WRITE.  Make the code less error prone
by moving checking for VM_WRITE into make_huge_pte(), and letting callers
only specify whether we should try making it writable.

A simple reproducer that longterm-pins the folios using liburing to then
mprotect(PROT_READ) the folios befor fork() [1] results in:

Before:
[FAIL] access should not have worked

After:
[PASS] access did not work as expected

[1] https://gitlab.com/davidhildenbrand/scratchspace/-/raw/main/reproducers/hugetlb-mkwrite-fork.c

This is rather a corner case, so stable might not be warranted.

Link: https://lkml.kernel.org/r/20241204153100.1967364-1-david@redhat.com


Fixes: 4eae4efa ("hugetlb: do early cow when page pinned on src mm")
Signed-off-by: default avatarDavid Hildenbrand <david@redhat.com>
Acked-by: default avatarPeter Xu <peterx@redhat.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Guillaume Morin <guillaume@morinfr.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent d0f14f7e

mm/hugetlb.c

+6 −12
Original line number Diff line number Diff line
@@ -5155,12 +5155,12 @@ const struct vm_operations_struct hugetlb_vm_ops = { 
};



static pte_t make_huge_pte(struct vm_area_struct *vma, struct page *page,

				int writable)

		bool try_mkwrite)

{

	pte_t entry;

	unsigned int shift = huge_page_shift(hstate_vma(vma));



	if (writable) {

	if (try_mkwrite && (vma->vm_flags & VM_WRITE)) {

		entry = huge_pte_mkwrite(huge_pte_mkdirty(mk_huge_pte(page,

					 vma->vm_page_prot)));

	} else {
@@ -5213,7 +5213,7 @@ static void 
hugetlb_install_folio(struct vm_area_struct *vma, pte_t *ptep, unsigned long addr,

		      struct folio *new_folio, pte_t old, unsigned long sz)

{

	pte_t newpte = make_huge_pte(vma, &new_folio->page, 1);

	pte_t newpte = make_huge_pte(vma, &new_folio->page, true);



	__folio_mark_uptodate(new_folio);

	hugetlb_add_new_anon_rmap(new_folio, vma, addr);
@@ -6249,8 +6249,7 @@ static vm_fault_t hugetlb_no_page(struct address_space *mapping, 
		hugetlb_add_new_anon_rmap(folio, vma, vmf->address);

	else

		hugetlb_add_file_rmap(folio);

	new_pte = make_huge_pte(vma, &folio->page, ((vma->vm_flags & VM_WRITE)

				&& (vma->vm_flags & VM_SHARED)));

	new_pte = make_huge_pte(vma, &folio->page, vma->vm_flags & VM_SHARED);

	/*

	 * If this pte was previously wr-protected, keep it wr-protected even

	 * if populated.
@@ -6582,7 +6581,6 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte, 
	spinlock_t *ptl;

	int ret = -ENOMEM;

	struct folio *folio;

	int writable;

	bool folio_in_pagecache = false;



	if (uffd_flags_mode_is(flags, MFILL_ATOMIC_POISON)) {
@@ -6736,12 +6734,8 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte, 
	 * For either: (1) CONTINUE on a non-shared VMA, or (2) UFFDIO_COPY

	 * with wp flag set, don't set pte write bit.

	 */

	if (wp_enabled || (is_continue && !vm_shared))

		writable = 0;

	else

		writable = dst_vma->vm_flags & VM_WRITE;



	_dst_pte = make_huge_pte(dst_vma, &folio->page, writable);

	_dst_pte = make_huge_pte(dst_vma, &folio->page,

				 !wp_enabled && !(is_continue && !vm_shared));

	/*

	 * Always mark UFFDIO_COPY page dirty; note that this may not be

	 * extremely important for hugetlbfs for now since swapping is not