mm/hugetlb: support FOLL_FORCE|FOLL_WRITE

Eric reported that PTRACE_POKETEXT fails when applications use hugetlb for
mapping text using huge pages.  Before commit 1d8d14641fd9 ("mm/hugetlb:
support write-faults in shared mappings"), PTRACE_POKETEXT worked by
accident, but it was buggy and silently ended up mapping pages writable
into the page tables even though VM_WRITE was not set.

In general, FOLL_FORCE|FOLL_WRITE does currently not work with hugetlb.
Let's implement FOLL_FORCE|FOLL_WRITE properly for hugetlb, such that what
used to work in the past by accident now properly works, allowing
applications using hugetlb for text etc.  to get properly debugged.

This change might also be required to implement uprobes support for
hugetlb [1].

[1] https://lore.kernel.org/lkml/ZiK50qob9yl5e0Xz@bender.morinfr.org/

Link: https://lkml.kernel.org/r/Z1NshNfWuzUCPebA@bender.morinfr.org
Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Eric Hagberg <ehagberg@janestreet.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index 3b75e631f36916da645e2ae2248cda9142b25752..00a1269cbee0a6db3ed1d6e2bcddd412abc516dd 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -596,6 +596,33 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
 }
 #endif /* CONFIG_HAVE_GUP_FAST */
 
+/* Common code for can_follow_write_* */
+static inline bool can_follow_write_common(struct page *page,
+               struct vm_area_struct *vma, unsigned int flags)
+{
+       /* Maybe FOLL_FORCE is set to override it? */
+       if (!(flags & FOLL_FORCE))
+               return false;
+
+       /* But FOLL_FORCE has no effect on shared mappings */
+       if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
+               return false;
+
+       /* ... or read-only private ones */
+       if (!(vma->vm_flags & VM_MAYWRITE))
+               return false;
+
+       /* ... or already writable ones that just need to take a write fault */
+       if (vma->vm_flags & VM_WRITE)
+               return false;
+
+       /*
+        * See can_change_pte_writable(): we broke COW and could map the page
+        * writable if we have an exclusive anonymous page ...
+        */
+       return page && PageAnon(page) && PageAnonExclusive(page);
+}
+
 static struct page *no_page_table(struct vm_area_struct *vma,
                                  unsigned int flags, unsigned long address)
 {
@@ -622,6 +649,18 @@ static struct page *no_page_table(struct vm_area_struct *vma,
 }
 
 #ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES
+/* FOLL_FORCE can write to even unwritable PUDs in COW mappings. */
+static inline bool can_follow_write_pud(pud_t pud, struct page *page,
+                                       struct vm_area_struct *vma,
+                                       unsigned int flags)
+{
+       /* If the pud is writable, we can write to the page. */
+       if (pud_write(pud))
+               return true;
+
+       return can_follow_write_common(page, vma, flags);
+}
+
 static struct page *follow_huge_pud(struct vm_area_struct *vma,
                                    unsigned long addr, pud_t *pudp,
                                    int flags, struct follow_page_context *ctx)
@@ -634,10 +673,11 @@ static struct page *follow_huge_pud(struct vm_area_struct *vma,
 
        assert_spin_locked(pud_lockptr(mm, pudp));
 
-       if ((flags & FOLL_WRITE) && !pud_write(pud))
+       if (!pud_present(pud))
                return NULL;
 
-       if (!pud_present(pud))
+       if ((flags & FOLL_WRITE) &&
+           !can_follow_write_pud(pud, pfn_to_page(pfn), vma, flags))
                return NULL;
 
        pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
@@ -686,27 +726,7 @@ static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
        if (pmd_write(pmd))
                return true;
 
-       /* Maybe FOLL_FORCE is set to override it? */
-       if (!(flags & FOLL_FORCE))
-               return false;
-
-       /* But FOLL_FORCE has no effect on shared mappings */
-       if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
-               return false;
-
-       /* ... or read-only private ones */
-       if (!(vma->vm_flags & VM_MAYWRITE))
-               return false;
-
-       /* ... or already writable ones that just need to take a write fault */
-       if (vma->vm_flags & VM_WRITE)
-               return false;
-
-       /*
-        * See can_change_pte_writable(): we broke COW and could map the page
-        * writable if we have an exclusive anonymous page ...
-        */
-       if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+       if (!can_follow_write_common(page, vma, flags))
                return false;
 
        /* ... and a write-fault isn't required for other reasons. */
@@ -807,27 +827,7 @@ static inline bool can_follow_write_pte(pte_t pte, struct page *page,
        if (pte_write(pte))
                return true;
 
-       /* Maybe FOLL_FORCE is set to override it? */
-       if (!(flags & FOLL_FORCE))
-               return false;
-
-       /* But FOLL_FORCE has no effect on shared mappings */
-       if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
-               return false;
-
-       /* ... or read-only private ones */
-       if (!(vma->vm_flags & VM_MAYWRITE))
-               return false;
-
-       /* ... or already writable ones that just need to take a write fault */
-       if (vma->vm_flags & VM_WRITE)
-               return false;
-
-       /*
-        * See can_change_pte_writable(): we broke COW and could map the page
-        * writable if we have an exclusive anonymous page ...
-        */
-       if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+       if (!can_follow_write_common(page, vma, flags))
                return false;
 
        /* ... and a write-fault isn't required for other reasons. */
@@ -1294,9 +1294,6 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
                if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
                        if (!(gup_flags & FOLL_FORCE))
                                return -EFAULT;
-                       /* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
-                       if (is_vm_hugetlb_page(vma))
-                               return -EFAULT;
                        /*
                         * We used to let the write,force case do COW in a
                         * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c9d8c6a1c03ce64e1a71d979ac6856f2381e8267..21de25546a254b3a642b40917de7788db787ce13 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5183,6 +5183,13 @@ static void set_huge_ptep_writable(struct vm_area_struct *vma,
                update_mmu_cache(vma, address, ptep);
 }
 
+static void set_huge_ptep_maybe_writable(struct vm_area_struct *vma,
+                                        unsigned long address, pte_t *ptep)
+{
+       if (vma->vm_flags & VM_WRITE)
+               set_huge_ptep_writable(vma, address, ptep);
+}
+
 bool is_hugetlb_entry_migration(pte_t pte)
 {
        swp_entry_t swp;
@@ -5828,13 +5835,6 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio,
        if (!unshare && huge_pte_uffd_wp(pte))
                return 0;
 
-       /*
-        * hugetlb does not support FOLL_FORCE-style write faults that keep the
-        * PTE mapped R/O such as maybe_mkwrite() would do.
-        */
-       if (WARN_ON_ONCE(!unshare && !(vma->vm_flags & VM_WRITE)))
-               return VM_FAULT_SIGSEGV;
-
        /* Let's take out MAP_SHARED mappings first. */
        if (vma->vm_flags & VM_MAYSHARE) {
                set_huge_ptep_writable(vma, vmf->address, vmf->pte);
@@ -5863,7 +5863,8 @@ retry_avoidcopy:
                        SetPageAnonExclusive(&old_folio->page);
                }
                if (likely(!unshare))
-                       set_huge_ptep_writable(vma, vmf->address, vmf->pte);
+                       set_huge_ptep_maybe_writable(vma, vmf->address,
+                                                    vmf->pte);
 
                delayacct_wpcopy_end();
                return 0;