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 <[email protected]> Signed-off-by: Ackerley Tng <[email protected]> --- .../testing/selftests/kvm/guest_memfd_test.c | 72 +++++++++++++++++++ 1 file changed, 72 insertions(+) diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c index 618c937f3c90f..d16341a4a315d 100644 --- a/tools/testing/selftests/kvm/guest_memfd_test.c +++ b/tools/testing/selftests/kvm/guest_memfd_test.c @@ -171,6 +171,77 @@ static void test_numa_allocation(int fd, size_t total_size) kvm_munmap(mem, total_size); } +static size_t getpmdsize(void) +{ + const char *path = "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"; + static size_t pmd_size = -1; + FILE *fp; + + if (pmd_size != -1) + return pmd_size; + + fp = fopen(path, "r"); + TEST_ASSERT(fp, "Couldn't open %s to read PMD size.", path); + + TEST_ASSERT_EQ(fscanf(fp, "%lu", &pmd_size), 1); + + TEST_ASSERT_EQ(fclose(fp), 0); + + return pmd_size; +} + +static void test_collapse(struct kvm_vm *vm, uint64_t flags) +{ + const size_t pmd_size = getpmdsize(); + char *mem; + off_t i; + int fd; + + fd = vm_create_guest_memfd(vm, pmd_size * 2, + GUEST_MEMFD_FLAG_MMAP | + GUEST_MEMFD_FLAG_INIT_SHARED); + + /* + * Use aligned address so that MADV_COLLAPSE will not be + * filtered out early in the collapsing routine. + */ +#define ALIGNED_ADDRESS ((void *)0x4000000000UL) + mem = mmap(ALIGNED_ADDRESS, pmd_size, PROT_READ | PROT_WRITE, + MAP_FIXED | MAP_SHARED, fd, 0); + TEST_ASSERT_EQ(mem, ALIGNED_ADDRESS); + + /* + * 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... + */ + TEST_ASSERT_EQ(madvise(mem, pmd_size, MADV_HUGEPAGE), 0); + + /* + * ... 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. + */ + TEST_ASSERT_EQ(madvise(mem, pmd_size, MADV_DONTNEED), 0); + READ_ONCE(mem[0]); + + kvm_munmap(mem, pmd_size); + kvm_close(fd); +} + static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_size) { const char val = 0xaa; @@ -370,6 +441,7 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags) gmem_test(mmap_supported, vm, flags); gmem_test(fault_overflow, vm, flags); gmem_test(numa_allocation, vm, flags); + test_collapse(vm, flags); } else { gmem_test(fault_private, vm, flags); } -- 2.53.0.273.g2a3d683680-goog
