[PATCH v5 1/2] s390/pci: Fix s390_mmio_read/write syscall page fault handling

Niklas Schnelle
2025-02-12T15:29:46Z

The s390 MMIO syscalls when using the classic PCI instructions do not
cause a page fault when follow_pfnmap_start() 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_pfnmap_start() with
fixup_user_page() and retrying the follow_pfnmap_start(). Also fix
a VM_READ vs VM_WRITE mixup in the read syscall.

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 46f99dc164ade4ca10f170cd66bdb648f92aa904..1997d9b7965df3b9b6019c7537028cd29d52fc13 100644
--- a/arch/s390/pci/pci_mmio.c
+++ b/arch/s390/pci/pci_mmio.c
@@ -175,8 +175,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
args.address = mmio_addr;
args.vma = vma;
ret = follow_pfnmap_start(&args);
- if (ret)
- goto out_unlock_mmap;
+ if (ret) {
+ fixup_user_fault(current->mm, mmio_addr, FAULT_FLAG_WRITE, NULL);
+ ret = follow_pfnmap_start(&args);
+ if (ret)
+ goto out_unlock_mmap;
+ }

io_addr = (void __iomem *)((args.pfn << PAGE_SHIFT) |
(mmio_addr & ~PAGE_MASK));
@@ -315,14 +319,18 @@ 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;

args.vma = vma;
args.address = mmio_addr;
ret = follow_pfnmap_start(&args);
- if (ret)
- goto out_unlock_mmap;
+ if (ret) {
+ fixup_user_fault(current->mm, mmio_addr, 0, NULL);
+ ret = follow_pfnmap_start(&args);
+ if (ret)
+ goto out_unlock_mmap;
+ }

io_addr = (void __iomem *)((args.pfn << PAGE_SHIFT) |
(mmio_addr & ~PAGE_MASK));

--
2.45.2