[PATCH v3 1/3] s390/pci: Fix s390_mmio_read/write syscall page fault handling

[Niklas Schnelle]

The s390 MMIO syscalls when using the classic PCI instructions do not
cause a page fault when follow_pte() fails due to the page not being
present. Besides being a general deficiency this breaks vfio-pci's mmap()
handling once VFIO_PCI_MMAP gets enabled as this lazily maps on first
access. Fix this by following a failed follow_pte() with
fixup_user_page() and retrying the follow_pte().

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Reviewed-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
---
 arch/s390/pci/pci_mmio.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
index 5398729bfe1b..80c21b1a101c 100644
--- a/arch/s390/pci/pci_mmio.c
+++ b/arch/s390/pci/pci_mmio.c
@@ -170,8 +170,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
 		goto out_unlock_mmap;
 
 	ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
-	if (ret)
-		goto out_unlock_mmap;
+	if (ret) {
+		fixup_user_fault(current->mm, mmio_addr, FAULT_FLAG_WRITE, NULL);
+		ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
+		if (ret)
+			goto out_unlock_mmap;
+	}
 
 	io_addr = (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
 			(mmio_addr & ~PAGE_MASK));
@@ -305,12 +309,16 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
 	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
 		goto out_unlock_mmap;
 	ret = -EACCES;
-	if (!(vma->vm_flags & VM_WRITE))
+	if (!(vma->vm_flags & VM_READ))
 		goto out_unlock_mmap;
 
 	ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
-	if (ret)
-		goto out_unlock_mmap;
+	if (ret) {
+		fixup_user_fault(current->mm, mmio_addr, 0, NULL);
+		ret = follow_pte(vma, mmio_addr, &ptep, &ptl);
+		if (ret)
+			goto out_unlock_mmap;
+	}
 
 	io_addr = (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
 			(mmio_addr & ~PAGE_MASK));

-- 
2.40.1