[PATCH v2 1/3] s390/pci: Fix s390_mmio_read/write syscall page fault handling

Niklas Schnelle
2024-05-23T11:10:36Z

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@nvidia.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
---
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 a90499c087f0..217defbcb4f1 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->vm_mm, mmio_addr, &ptep, &ptl);
- if (ret)
- goto out_unlock_mmap;
+ if (ret) {
+ fixup_user_fault(vma->vm_mm, mmio_addr, FAULT_FLAG_WRITE, NULL);
+ ret = follow_pte(vma->vm_mm, 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->vm_mm, mmio_addr, &ptep, &ptl);
- if (ret)
- goto out_unlock_mmap;
+ if (ret) {
+ fixup_user_fault(vma->vm_mm, mmio_addr, 0, NULL);
+ ret = follow_pte(vma->vm_mm, 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