[PATCH mm-unstable v1 04/20] mm: add early FAULT_FLAG_UNSHARE consistency checks

David Hildenbrand
2022-11-16T10:34:38Z

For now, FAULT_FLAG_UNSHARE only applies to anonymous pages, which
implies a COW mapping. Let's hide FAULT_FLAG_UNSHARE early if we're not
dealing with a COW mapping, such that we treat it like a read fault as
documented and don't have to worry about the flag throughout all fault
handlers.

While at it, centralize the check for mutual exclusion of
FAULT_FLAG_UNSHARE and FAULT_FLAG_WRITE and just drop the check that
either flag is set in the WP handler.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/huge_memory.c | 3 ---
mm/hugetlb.c | 5 -----
mm/memory.c | 23 ++++++++++++++++++++---
3 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ed12cd3acbfd..68d00196b519 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1267,9 +1267,6 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
VM_BUG_ON_VMA(!vma->anon_vma, vma);

- VM_BUG_ON(unshare && (vmf->flags & FAULT_FLAG_WRITE));
- VM_BUG_ON(!unshare && !(vmf->flags & FAULT_FLAG_WRITE));
-
if (is_huge_zero_pmd(orig_pmd))
goto fallback;

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1de986c62976..383b26069b33 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5314,9 +5314,6 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long haddr = address & huge_page_mask(h);
struct mmu_notifier_range range;

- VM_BUG_ON(unshare && (flags & FOLL_WRITE));
- VM_BUG_ON(!unshare && !(flags & FOLL_WRITE));
-
/*
* hugetlb does not support FOLL_FORCE-style write faults that keep the
* PTE mapped R/O such as maybe_mkwrite() would do.
@@ -5326,8 +5323,6 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,

/* Let's take out MAP_SHARED mappings first. */
if (vma->vm_flags & VM_MAYSHARE) {
- if (unlikely(unshare))
- return 0;
set_huge_ptep_writable(vma, haddr, ptep);
return 0;
}
diff --git a/mm/memory.c b/mm/memory.c
index 2d453736f87c..e014435a87db 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3344,9 +3344,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
struct vm_area_struct *vma = vmf->vma;
struct folio *folio;

- VM_BUG_ON(unshare && (vmf->flags & FAULT_FLAG_WRITE));
- VM_BUG_ON(!unshare && !(vmf->flags & FAULT_FLAG_WRITE));
-
if (likely(!unshare)) {
if (userfaultfd_pte_wp(vma, *vmf->pte)) {
pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -5161,6 +5158,22 @@ static void lru_gen_exit_fault(void)
}
#endif /* CONFIG_LRU_GEN */

+static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
+ unsigned int *flags)
+{
+ if (unlikely(*flags & FAULT_FLAG_UNSHARE)) {
+ if (WARN_ON_ONCE(*flags & FAULT_FLAG_WRITE))
+ return VM_FAULT_SIGSEGV;
+ /*
+ * FAULT_FLAG_UNSHARE only applies to COW mappings. Let's
+ * just treat it like an ordinary read-fault otherwise.
+ */
+ if (!is_cow_mapping(vma->vm_flags))
+ *flags &= ~FAULT_FLAG_UNSHARE;
+ }
+ return 0;
+}
+
/*
* By the time we get here, we already hold the mm semaphore
*
@@ -5177,6 +5190,10 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
count_vm_event(PGFAULT);
count_memcg_event_mm(vma->vm_mm, PGFAULT);

+ ret = sanitize_fault_flags(vma, &flags);
+ if (ret)
+ return ret;
+
if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
flags & FAULT_FLAG_INSTRUCTION,
flags & FAULT_FLAG_REMOTE))
--
2.38.1