[PATCH v1] vfio/pci: pass FAULT_FLAG_WRITE to vmf_insert_pfn_pmd/pud()

[Mina Almasry <almasrymina@google.com>]

vfio_pci_vmf_insert_pfn() unconditionally passes write=false to
vmf_insert_pfn_pmd() and vmf_insert_pfn_pud().

In insert_pmd() and insert_pud(), write=false skips marking newly
inserted PMD/PUD entries dirty/young via pmd_mkyoung(pmd_mkdirty()) and
maybe_pmd_mkwrite(). Furthermore, when vma_wants_writenotify() is active
on a shared writable VMA (for example, after CLEAR_REFS_SOFT_DIRTY
clears VM_SOFTDIRTY and calls vma_set_page_prot()), vm_page_prot is
downgraded to read-only so insert_pmd()/insert_pud() with write=false
installs a write-protected huge PMD/PUD. Subsequent write faults enter
wp_huge_pmd() or wp_huge_pud(), which invoke vma->vm_ops->huge_fault()
to upgrade the entry in place. Because vfio_pci_vmf_insert_pfn() passes
write=false, insert_pmd() and insert_pud() see !pmd_none(*pmd) with
!write, do nothing, and return VM_FAULT_NOPAGE without setting the write
bit or splitting the huge page, causing an infinite write page-fault
loop.

Pass vmf->flags & FAULT_FLAG_WRITE to vmf_insert_pfn_pmd() and
vmf_insert_pfn_pud() so write faults properly mark huge PFNMAP entries
dirty and writable.

Fixes: f9e54c3a2f5b ("vfio/pci: implement huge_fault support")
Cc: stable@vger.kernel.org
Cc: Ankit Agrawal <ankita@nvidia.com>
Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Assisted-by: LLM
Signed-off-by: Mina Almasry <almasrymina@google.com>
---
 drivers/vfio/pci/vfio_pci_core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index fcbb2da860e2..204d959086e9 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1745,6 +1745,8 @@ vm_fault_t vfio_pci_vmf_insert_pfn(struct vfio_pci_core_device *vdev,
 				   unsigned long pfn,
 				   unsigned int order)
 {
+	bool write = vmf->flags & FAULT_FLAG_WRITE;
+
 	lockdep_assert_held_read(&vdev->memory_lock);
 
 	if (vdev->pm_runtime_engaged || !__vfio_pci_memory_enabled(vdev))
@@ -1754,10 +1756,10 @@ vm_fault_t vfio_pci_vmf_insert_pfn(struct vfio_pci_core_device *vdev,
 		return vmf_insert_pfn(vmf->vma, vmf->address, pfn);
 
 	if (IS_ENABLED(CONFIG_ARCH_SUPPORTS_PMD_PFNMAP) && order == PMD_ORDER)
-		return vmf_insert_pfn_pmd(vmf, pfn, false);
+		return vmf_insert_pfn_pmd(vmf, pfn, write);
 
 	if (IS_ENABLED(CONFIG_ARCH_SUPPORTS_PUD_PFNMAP) && order == PUD_ORDER)
-		return vmf_insert_pfn_pud(vmf, pfn, false);
+		return vmf_insert_pfn_pud(vmf, pfn, write);
 
 	return VM_FAULT_FALLBACK;
 }

base-commit: 076d7e989cb69f6ded7d60f2a080eb909930061b
-- 
2.56.0.257.gf7497c7d27-goog