linux-kernel - [RFC PATCH v1 32/37] KVM: selftests: Check fd/flags provided to mmap() when setting up memslot

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Message-ID: <a3126618d7cb353faad7b231e70c2b732498f449.1760731772.git.ackerleytng@google.com>
Date: Fri, 17 Oct 2025 13:12:13 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
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Subject: [RFC PATCH v1 32/37] KVM: selftests: Check fd/flags provided to
 mmap() when setting up memslot

From: Sean Christopherson <seanjc@...gle.com>

Check that a valid fd provided to mmap() must be accompanied by MAP_SHARED.

With an invalid fd (usually used for anonymous mappings), there are no
constraints on mmap() flags.

Add this check to make sure that when a guest_memfd is used as region->fd,
the flag provided to mmap() will include MAP_SHARED.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
[Rephrase assertion message.]
Signed-off-by: Ackerley Tng <ackerleytng@...gle.com>
---
 tools/testing/selftests/kvm/lib/kvm_util.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
index cb73566fdf153..8603bd5c705ed 100644
--- a/tools/testing/selftests/kvm/lib/kvm_util.c
+++ b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -1057,6 +1057,9 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
 		region->fd = kvm_memfd_alloc(region->mmap_size,
 					     src_type == VM_MEM_SRC_SHARED_HUGETLB);
 
+	TEST_ASSERT(region->fd == -1 || backing_src_is_shared(src_type),
+		    "A valid fd provided to mmap() must be accompanied by MAP_SHARED.");
+
 	mmap_offset = flags & KVM_MEM_GUEST_MEMFD ? gmem_offset : 0;
 	region->mmap_start = __kvm_mmap(region->mmap_size, PROT_READ | PROT_WRITE,
 					vm_mem_backing_src_alias(src_type)->flag,
-- 
2.51.0.858.gf9c4a03a3a-goog