linux-kernel - Re: [PATCH 6/6] KVM: selftests: Verify that faulting in private guest

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Message-ID: <CA+EHjTxhyPRVbWWK6h86Wfw2oZbOVSekwAjKNxW-Ab=qaOhk3g@mail.gmail.com>
Date: Mon, 29 Sep 2025 10:24:34 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>, 
	Ackerley Tng <ackerleytng@...gle.com>
Subject: Re: [PATCH 6/6] KVM: selftests: Verify that faulting in private
 guest_memfd memory fails

On Fri, 26 Sept 2025 at 17:31, Sean Christopherson <seanjc@...gle.com> wrote:
>
> Add a guest_memfd testcase to verify that faulting in private memory gets
> a SIGBUS.  For now, test only the case where memory is private by default
> since KVM doesn't yet support in-place conversion.
>
> Cc: Ackerley Tng <ackerleytng@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>


Reviewed-by: Fuad Tabba <tabba@...gle.com>
Tested-by: Fuad Tabba <tabba@...gle.com>

Cheers,
/fuad


> ---
>  .../testing/selftests/kvm/guest_memfd_test.c  | 62 ++++++++++++++-----
>  1 file changed, 46 insertions(+), 16 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/guest_memfd_test.c b/tools/testing/selftests/kvm/guest_memfd_test.c
> index 5dd40b77dc07..b5a631aca933 100644
> --- a/tools/testing/selftests/kvm/guest_memfd_test.c
> +++ b/tools/testing/selftests/kvm/guest_memfd_test.c
> @@ -40,17 +40,26 @@ static void test_file_read_write(int fd, size_t total_size)
>                     "pwrite on a guest_mem fd should fail");
>  }
>
> -static void test_mmap_supported(int fd, size_t total_size)
> +static void *test_mmap_common(int fd, size_t size)
>  {
> -       const char val = 0xaa;
> -       char *mem;
> -       size_t i;
> -       int ret;
> +       void *mem;
>
> -       mem = mmap(NULL, total_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
> +       mem = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
>         TEST_ASSERT(mem == MAP_FAILED, "Copy-on-write not allowed by guest_memfd.");
>
> -       mem = kvm_mmap(total_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
> +       mem = kvm_mmap(size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
> +
> +       return mem;
> +}
> +
> +static void test_mmap_supported(int fd, size_t total_size)
> +{
> +       const char val = 0xaa;
> +       char *mem;
> +       size_t i;
> +       int ret;
> +
> +       mem = test_mmap_common(fd, total_size);
>
>         memset(mem, val, total_size);
>         for (i = 0; i < total_size; i++)
> @@ -78,31 +87,47 @@ void fault_sigbus_handler(int signum)
>         siglongjmp(jmpbuf, 1);
>  }
>
> -static void test_fault_overflow(int fd, size_t total_size)
> +static void *test_fault_sigbus(int fd, size_t size)
>  {
>         struct sigaction sa_old, sa_new = {
>                 .sa_handler = fault_sigbus_handler,
>         };
> -       size_t map_size = total_size * 4;
> -       const char val = 0xaa;
> -       char *mem;
> -       size_t i;
> +       void *mem;
>
> -       mem = kvm_mmap(map_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd);
> +       mem = test_mmap_common(fd, size);
>
>         sigaction(SIGBUS, &sa_new, &sa_old);
>         if (sigsetjmp(jmpbuf, 1) == 0) {
> -               memset(mem, 0xaa, map_size);
> +               memset(mem, 0xaa, size);
>                 TEST_ASSERT(false, "memset() should have triggered SIGBUS.");
>         }
>         sigaction(SIGBUS, &sa_old, NULL);
>
> +       return mem;
> +}
> +
> +static void test_fault_overflow(int fd, size_t total_size)
> +{
> +       size_t map_size = total_size * 4;
> +       const char val = 0xaa;
> +       char *mem;
> +       size_t i;
> +
> +       mem = test_fault_sigbus(fd, map_size);
> +
>         for (i = 0; i < total_size; i++)
>                 TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);
>
>         kvm_munmap(mem, map_size);
>  }
>
> +static void test_fault_private(int fd, size_t total_size)
> +{
> +       void *mem = test_fault_sigbus(fd, total_size);
> +
> +       kvm_munmap(mem, total_size);
> +}
> +
>  static void test_mmap_not_supported(int fd, size_t total_size)
>  {
>         char *mem;
> @@ -274,9 +299,12 @@ static void __test_guest_memfd(struct kvm_vm *vm, uint64_t flags)
>
>         gmem_test(file_read_write, vm, flags);
>
> -       if (flags & GUEST_MEMFD_FLAG_MMAP) {
> +       if (flags & GUEST_MEMFD_FLAG_MMAP &&
> +           flags & GUEST_MEMFD_FLAG_DEFAULT_SHARED) {
>                 gmem_test(mmap_supported, vm, flags);
>                 gmem_test(fault_overflow, vm, flags);
> +       } else if (flags & GUEST_MEMFD_FLAG_MMAP) {
> +               gmem_test(fault_private, vm, flags);
>         } else {
>                 gmem_test(mmap_not_supported, vm, flags);
>         }
> @@ -294,9 +322,11 @@ static void test_guest_memfd(unsigned long vm_type)
>
>         __test_guest_memfd(vm, 0);
>
> -       if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP))
> +       if (vm_check_cap(vm, KVM_CAP_GUEST_MEMFD_MMAP)) {
> +               __test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP);
>                 __test_guest_memfd(vm, GUEST_MEMFD_FLAG_MMAP |
>                                        GUEST_MEMFD_FLAG_DEFAULT_SHARED);
> +       }
>
>         kvm_vm_free(vm);
>  }
> --
> 2.51.0.536.g15c5d4f767-goog
>