[PATCH 5.4 STABLE] s390/pci_mmio: fully validate the VMA before calling follow_pte()

[David Hildenbrand]

commit a8b92b8c1eac8d655a97b1e90f4d83c25d9b9a18 upstream.

Note: We don't have vma_lookup() in the 5.4-stable tree, so perform the VMA
check manually.

We should not walk/touch page tables outside of VMA boundaries when
holding only the mmap sem in read mode. Evil user space can modify the
VMA layout just before this function runs and e.g., trigger races with
page table removal code since commit dd2283f2605e ("mm: mmap: zap pages
with read mmap_sem in munmap").

find_vma() does not check if the address is >= the VMA start address;
use vma_lookup() instead.

Reviewed-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 arch/s390/pci/pci_mmio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
index 020a2c514d96..921f0fc12f1f 100644
--- a/arch/s390/pci/pci_mmio.c
+++ b/arch/s390/pci/pci_mmio.c
@@ -128,7 +128,7 @@ static long get_pfn(unsigned long user_addr, unsigned long access,
 	down_read(&current->mm->mmap_sem);
 	ret = -EINVAL;
 	vma = find_vma(current->mm, user_addr);
-	if (!vma)
+	if (!vma || user_addr < vma->vm_start)
 		goto out;
 	ret = -EACCES;
 	if (!(vma->vm_flags & access))
-- 
2.31.1