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: 4eae4efa2c29 ("hugetlb: do early cow when page pinned on src mm")
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Guillaume Morin <guillaume@morinfr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index ac275a8864e0dda22656c0ca4b2a6297544efde8..c9d8c6a1c03ce64e1a71d979ac6856f2381e8267 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -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