[PATCH v2 0/2] Test MADV_COLLAPSE on guest_memfd

[PATCH v2 0/2] Test MADV_COLLAPSE on guest_memfd

[Ackerley Tng]

syzkaller identified that khugepaged, operating on guest_memfd memory,
could cause guest_memfd folios to get collapsed, leading to a WARNing
during fault [1].

Add selftest to guard against similar regressions.

Changes in v2:

+ Found get_trans_hugepagesz(), which I should have used instead of
  adding getpmdsize()
+ Extended tools/testing/selftests/kvm/include/kvm_syscalls.h to add
  kvm_madvise()
+ Removed the magic constant address and explained alignment requirements in
  comments
+ Refactored gmem_test() macro to expose __gmem_test(), which allows custom
  sized guest_memfds for tests. Sean, I didn't add the gmem_test_huge_pmd()
  since I'm guessing a test requiring a pmd_sized guest_memfd would probably be
  once-off.

PATCH v1 [3] was sent in reply to the fix [2].

[1] https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44
[2] https://lore.kernel.org/all/20260214001535.435626-1-kartikey406@gmail.com/
[3] https://lore.kernel.org/all/20260217014402.2554832-1-ackerleytng@google.com/

Ackerley Tng (2):
  KVM: selftests: Wrap madvise() to assert success
  KVM: selftests: Test MADV_COLLAPSE on guest_memfd

 .../testing/selftests/kvm/guest_memfd_test.c  | 70 ++++++++++++++++++-
 .../selftests/kvm/include/kvm_syscalls.h      |  1 +
 2 files changed, 68 insertions(+), 3 deletions(-)


base-commit: a95f71ad3e2e224277508e006580c333d0a5fe36
prerequisite-patch-id: e001eecc9215dc0ed28546936f86a5a09e57141e
--
2.53.0.345.g96ddfc5eaa-goog

[PATCH v2 1/2] KVM: selftests: Wrap madvise() to assert success

[Ackerley Tng]

Extend kvm_syscalls.h to wrap madvise() to assert success. This will be
used in the next patch.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 tools/testing/selftests/kvm/include/kvm_syscalls.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/kvm/include/kvm_syscalls.h b/tools/testing/selftests/kvm/include/kvm_syscalls.h
index d4e613162bba9..843c9904c46f6 100644
--- a/tools/testing/selftests/kvm/include/kvm_syscalls.h
+++ b/tools/testing/selftests/kvm/include/kvm_syscalls.h
@@ -77,5 +77,6 @@ __KVM_SYSCALL_DEFINE(munmap, 2, void *, mem, size_t, size);
 __KVM_SYSCALL_DEFINE(close, 1, int, fd);
 __KVM_SYSCALL_DEFINE(fallocate, 4, int, fd, int, mode, loff_t, offset, loff_t, len);
 __KVM_SYSCALL_DEFINE(ftruncate, 2, unsigned int, fd, off_t, length);
+__KVM_SYSCALL_DEFINE(madvise, 3, void *, addr, size_t, length, int, advice);
 
 #endif /* SELFTEST_KVM_SYSCALLS_H */
-- 
2.53.0.345.g96ddfc5eaa-goog

[PATCH v2 2/2] KVM: selftests: Test MADV_COLLAPSE on guest_memfd

[Ackerley Tng]

guest_memfd only supports PAGE_SIZE pages, and khugepaged or MADV_COLLAPSE
collapsing pages may result in private memory regions being mapped into
host page tables.

Add test to verify that MADV_COLLAPSE fails on guest_memfd folios, and any
subsequent usage of guest_memfd memory faults in PAGE_SIZE folios. Running
this test should not result in any memory failure logs or kernel WARNings.

This selftest was added as a result of a syzbot-reported issue where
khugepaged operating on guest_memfd memory with MADV_HUGEPAGE caused the
collapse of folios, which then subsequently resulted in a WARNing.

Link: https://syzkaller.appspot.com/bug?extid=33a04338019ac7e43a44
Suggested-by: David Hildenbrand <david@kernel.org>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
 .../testing/selftests/kvm/guest_memfd_test.c  | 70 ++++++++++++++++++-
 1 file changed, 67 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
index 618c937f3c90f..0edbc7cf6c1ad 100644
--- a/tools/testing/selftests/kvm/guest_memfd_test.c
+++ b/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -171,6 +171,64 @@ static void test_numa_allocation(int fd, size_t total_size)
 	kvm_munmap(mem, total_size);
 }
 
+static void test_collapse(int fd, uint64_t flags)
+{
+	const size_t pmd_size = get_trans_hugepagesz();
+	void *reserved_addr;
+	void *aligned_addr;
+	char *mem;
+	off_t i;
+
+	/*
+	 * To even reach the point where the guest_memfd folios will
+	 * get collapsed, both the userspace address and the offset
+	 * within the guest_memfd have to be aligned to pmd_size.
+	 *
+	 * To achieve that alignment, reserve virtual address space
+	 * with regular mmap, then use MAP_FIXED to allocate memory
+	 * from a pmd_size-aligned offset (0) at a known, available
+	 * virtual address.
+	 */
+	reserved_addr = kvm_mmap(pmd_size * 2, PROT_NONE,
+				 MAP_PRIVATE | MAP_ANONYMOUS, -1);
+	aligned_addr = align_ptr_up(reserved_addr, pmd_size);
+
+	mem = mmap(aligned_addr, pmd_size, PROT_READ | PROT_WRITE,
+		   MAP_FIXED | MAP_SHARED, fd, 0);
+	TEST_ASSERT(IS_ALIGNED((u64)mem, pmd_size),
+		    "Userspace address must be aligned to PMD size.");
+
+	/*
+	 * Use reads to populate page table to avoid setting dirty
+	 * flag on page.
+	 */
+	for (i = 0; i < pmd_size; i += getpagesize())
+		READ_ONCE(mem[i]);
+
+	/*
+	 * Advising the use of huge pages in guest_memfd should be
+	 * fine...
+	 */
+	kvm_madvise(mem, pmd_size, MADV_HUGEPAGE);
+
+	/*
+	 * ... but collapsing folios must not be supported to avoid
+	 * mapping beyond shared ranges into host userspace page
+	 * tables.
+	 */
+	TEST_ASSERT_EQ(madvise(mem, pmd_size, MADV_COLLAPSE), -1);
+	TEST_ASSERT_EQ(errno, EINVAL);
+
+	/*
+	 * Removing from host page tables and re-faulting should be
+	 * fine; should not end up faulting in a collapsed/huge folio.
+	 */
+	kvm_madvise(mem, pmd_size, MADV_DONTNEED);
+	READ_ONCE(mem[0]);
+
+	kvm_munmap(reserved_addr, pmd_size * 2);
+}
+
 static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_size)
 {
 	const char val = 0xaa;
@@ -350,14 +408,17 @@ static void test_guest_memfd_flags(struct kvm_vm *vm)
 	}
 }
 
-#define gmem_test(__test, __vm, __flags)				\
+#define __gmem_test(__test, __vm, __flags, __gmem_size)			\
 do {									\
-	int fd = vm_create_guest_memfd(__vm, page_size * 4, __flags);	\
+	int fd = vm_create_guest_memfd(__vm, __gmem_size, __flags);	\
 									\
-	test_##__test(fd, page_size * 4);				\
+	test_##__test(fd, __gmem_size);					\
 	close(fd);							\
 } while (0)
 
+#define gmem_test(__test, __vm, __flags) 				\
+	__gmem_test(__test, __vm, __flags, page_size * 4)
+
 static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
 {
 	test_create_guest_memfd_multiple(vm);
@@ -367,9 +428,12 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
 
 	if (flags & GUEST_MEMFD_FLAG_MMAP) {
 		if (flags & GUEST_MEMFD_FLAG_INIT_SHARED) {
+			size_t pmd_size = get_trans_hugepagesz();
+
 			gmem_test(mmap_supported, vm, flags);
 			gmem_test(fault_overflow, vm, flags);
 			gmem_test(numa_allocation, vm, flags);
+			__gmem_test(collapse, vm, flags, pmd_size);
 		} else {
 			gmem_test(fault_private, vm, flags);
 		}
-- 
2.53.0.345.g96ddfc5eaa-goog