KVM: fix async page fault working for readonly mapping

If we map a readonly memory space from host to guest and the page is not currently mapped in the host, we will get a fault-pfn and async is not allowed, then the vm will crash

The reason is only writable vma can be allowed to be async in current code

Signed-off-by Xiao Guangrong <[email protected]>

---
 virt/kvm/kvm_main.c |   19 +++++++++++++++++--
 1 files changed, 17 insertions(+), 2 deletions(-)

virt/kvm/kvm_main.c +17 -2

index 6bd34a6..b6c8962 100644
@@ -1052,6 +1052,21 @@ static inline int check_user_page_hwpoison(unsigned long addr)
 	return rc == -EHWPOISON;
 }
 
+static bool vma_is_avalid(struct vm_area_struct *vma, bool write_fault)
+{
+	if (write_fault) {
+		if (unlikely(!(vma->vm_flags & VM_WRITE)))
+			return false;
+
+		return true;
+	}
+
+	if (unlikely(!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))))
+		return false;
+
+	return true;
+}
+
 static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
 			bool *async, bool write_fault, bool *writable)
 {

@@ -1075,7 +1090,6 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
 
 		if (writable)
 			*writable = write_fault;
-
 		if (async) {
 			down_read(&current->mm->mmap_sem);
 			npages = get_user_page_nowait(current, current->mm,

@@ -1122,8 +1136,9 @@ static pfn_t hva_to_pfn(struct kvm *kvm, unsigned long addr, bool atomic,
 				vma->vm_pgoff;
 			BUG_ON(!kvm_is_mmio_pfn(pfn));
 		} else {
-			if (async && (vma->vm_flags & VM_WRITE))
+			if (async && vma_is_avalid(vma, write_fault))
 				*async = true;
+
 			pfn = get_fault_pfn();
 		}
 		up_read(&current->mm->mmap_sem);

Signature · 7 lines

1.7.7.6

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Quoted 4 lines · Xiao Guangrong

If we map a readonly memory space from host to guest and the page is not currently mapped in the host, we will get a fault-pfn and async is not allowed, then the vm will crash

Why would we want to map a readonly memory space from host to guest? We may want to do it to support memory semantics on read and mmio on write, but do not right now unless something changed while I was not looking.

Quoted 5 lines · Gleb Natapov

Quoted 4 lines · Xiao Guangrong

If we map a readonly memory space from host to guest and the page is not currently mapped in the host, we will get a fault-pfn and async is not allowed, then the vm will crash

Quoted 4 lines

Why would we want to map a readonly memory space from host to guest? We may want to do it to support memory semantics on read and mmio on write, but do not right now unless something changed while I was not looking.

Some test cases in kvm-unit-tests and the benchmark i am writing for KVM need map the function on host to guest.

Quoted 13 lines · Xiao Guangrong

Quoted 9 lines · Gleb Natapov

Quoted 4 lines · Xiao Guangrong

If we map a readonly memory space from host to guest and the page is not currently mapped in the host, we will get a fault-pfn and async is not allowed, then the vm will crash

Why would we want to map a readonly memory space from host to guest? We may want to do it to support memory semantics on read and mmio on write, but do not right now unless something changed while I was not looking.

Quoted 2 lines

Some test cases in kvm-unit-tests and the benchmark i am writing for KVM need map the function on host to guest.

Or ROM. Or read-only mappings of IVSHMEM (which don't exist yet).

Quoted 14 lines · Marcelo Tosatti

Quoted 13 lines · Xiao Guangrong

Quoted 9 lines · Gleb Natapov

Quoted 4 lines · Xiao Guangrong

If we map a readonly memory space from host to guest and the page is not currently mapped in the host, we will get a fault-pfn and async is not allowed, then the vm will crash

Why would we want to map a readonly memory space from host to guest? We may want to do it to support memory semantics on read and mmio on write, but do not right now unless something changed while I was not looking.

Quoted 4 lines

Quoted 2 lines

Some test cases in kvm-unit-tests and the benchmark i am writing for KVM need map the function on host to guest.

Or ROM. Or read-only mappings of IVSHMEM (which don't exist yet).

True. KVM should ignore writes to such areas, not kill a guest. Is this how the code works today?

Quoted 15 lines · Gleb Natapov

Quoted 14 lines · Marcelo Tosatti

Quoted 13 lines · Xiao Guangrong

Quoted 9 lines · Gleb Natapov

Quoted 4 lines · Xiao Guangrong

If we map a readonly memory space from host to guest and the page is not currently mapped in the host, we will get a fault-pfn and async is not allowed, then the vm will crash

Why would we want to map a readonly memory space from host to guest? We may want to do it to support memory semantics on read and mmio on write, but do not right now unless something changed while I was not looking.

Quoted 7 lines

Quoted 4 lines

Quoted 2 lines

Some test cases in kvm-unit-tests and the benchmark i am writing for KVM need map the function on host to guest.

Or ROM. Or read-only mappings of IVSHMEM (which don't exist yet).

True. KVM should ignore writes to such areas, not kill a guest. Is this how the code works today?

Right now qemu maps ROM as RAM. There is no way to tell kvm that something is ROM (or ROMD).

There are two options for that:
- mprotect() the ROM, and teach kvm about read-only areas (this patch); but that doesn't work if we have a read-only and a writable alias of the same area
- add a flag indicating that an area is ROM or ROMD

I prefer the latter, because of the alias issue.