Re: [PATCH v8 7/7] selftest: test system mappings are sealed.

[Jeff Xu <jeffxu@chromium.org>]

On Mon, Mar 3, 2025 at 9:01 AM Kees Cook <kees@kernel.org> wrote:
>
> On Mon, Mar 03, 2025 at 05:09:21AM +0000, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@chromium.org>
> >
> > Add sysmap_is_sealed.c to test system mappings are sealed.
> >
> > Note: CONFIG_MSEAL_SYSTEM_MAPPINGS must be set, as indicated in
> > config file.
> >
> > Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> > ---
> >  .../mseal_system_mappings/.gitignore          |   2 +
> >  .../selftests/mseal_system_mappings/Makefile  |   6 +
> >  .../selftests/mseal_system_mappings/config    |   1 +
> >  .../mseal_system_mappings/sysmap_is_sealed.c  | 113 ++++++++++++++++++
> >  4 files changed, 122 insertions(+)
> >  create mode 100644 tools/testing/selftests/mseal_system_mappings/.gitignore
> >  create mode 100644 tools/testing/selftests/mseal_system_mappings/Makefile
> >  create mode 100644 tools/testing/selftests/mseal_system_mappings/config
> >  create mode 100644 tools/testing/selftests/mseal_system_mappings/sysmap_is_sealed.c
> >
> > diff --git a/tools/testing/selftests/mseal_system_mappings/.gitignore b/tools/testing/selftests/mseal_system_mappings/.gitignore
> > new file mode 100644
> > index 000000000000..319c497a595e
> > --- /dev/null
> > +++ b/tools/testing/selftests/mseal_system_mappings/.gitignore
> > @@ -0,0 +1,2 @@
> > +# SPDX-License-Identifier: GPL-2.0-only
> > +sysmap_is_sealed
> > diff --git a/tools/testing/selftests/mseal_system_mappings/Makefile b/tools/testing/selftests/mseal_system_mappings/Makefile
> > new file mode 100644
> > index 000000000000..2b4504e2f52f
> > --- /dev/null
> > +++ b/tools/testing/selftests/mseal_system_mappings/Makefile
> > @@ -0,0 +1,6 @@
> > +# SPDX-License-Identifier: GPL-2.0-only
> > +CFLAGS += -std=c99 -pthread -Wall $(KHDR_INCLUDES)
> > +
> > +TEST_GEN_PROGS := sysmap_is_sealed
> > +
> > +include ../lib.mk
> > diff --git a/tools/testing/selftests/mseal_system_mappings/config b/tools/testing/selftests/mseal_system_mappings/config
> > new file mode 100644
> > index 000000000000..675cb9f37b86
> > --- /dev/null
> > +++ b/tools/testing/selftests/mseal_system_mappings/config
> > @@ -0,0 +1 @@
> > +CONFIG_MSEAL_SYSTEM_MAPPINGS=y
> > diff --git a/tools/testing/selftests/mseal_system_mappings/sysmap_is_sealed.c b/tools/testing/selftests/mseal_system_mappings/sysmap_is_sealed.c
> > new file mode 100644
> > index 000000000000..c1e93794a58b
> > --- /dev/null
> > +++ b/tools/testing/selftests/mseal_system_mappings/sysmap_is_sealed.c
> > @@ -0,0 +1,113 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * test system mappings are sealed when
> > + * KCONFIG_MSEAL_SYSTEM_MAPPINGS=y
> > + */
> > +
> > +#define _GNU_SOURCE
> > +#include <stdio.h>
> > +#include <errno.h>
> > +#include <unistd.h>
> > +#include <string.h>
> > +#include <stdbool.h>
> > +
> > +#include "../kselftest.h"
> > +#include "../kselftest_harness.h"
> > +
> > +#define VDSO_NAME "[vdso]"
> > +#define VVAR_NAME "[vvar]"
> > +#define VVAR_VCLOCK_NAME "[vvar_vclock]"
> > +#define UPROBES_NAME "[uprobes]"
> > +#define SIGPAGE_NAME "[sigpage]"
> > +#define VECTORS_NAME "[vectors]"
>
> These are only ever used once, and it feels like having them spelled out
> right in the variant definitions would be more readable, but I'm not
> sure I feel strongly enough about it to say it should be changed.
> They're available via "variant->name" as well, which makes it unlikely
> the macros will be used anywhere in the future? Maybe you have plans for
> them. :)
No plan for reuse them in other code, will move to Variant in v9.

>
> > +#define VMFLAGS "VmFlags:"
>
> This one gets a strlen() on it, so it feels better to have a macro.
>
Ok, thanks for the reasoning.

> > +#define MSEAL_FLAGS "sl"
> > +#define MAX_LINE_LEN 512
> > +
> > +bool has_mapping(char *name, FILE *maps)
> > +{
> > +     char line[MAX_LINE_LEN];
> > +
> > +     while (fgets(line, sizeof(line), maps)) {
> > +             if (strstr(line, name))
> > +                     return true;
> > +     }
> > +
> > +     return false;
> > +}
> > +
> > +bool mapping_is_sealed(char *name, FILE *maps)
> > +{
> > +     char line[MAX_LINE_LEN];
> > +
> > +     while (fgets(line, sizeof(line), maps)) {
> > +             if (!strncmp(line, VMFLAGS, strlen(VMFLAGS))) {
> > +                     if (strstr(line, MSEAL_FLAGS))
> > +                             return true;
> > +
> > +                     return false;
> > +             }
> > +     }
> > +
> > +     return false;
> > +}
> > +
> > +FIXTURE(basic) {
> > +     FILE *maps;
> > +};
> > +
> > +FIXTURE_SETUP(basic)
> > +{
> > +     self->maps = fopen("/proc/self/smaps", "r");
> > +     if (!self->maps)
> > +             SKIP(return, "Could not open /proc/self/smap, errno=%d",
> > +                     errno);
>
> Good SKIP usage, though I wonder if not having /proc should be a full
> blown failure?
>
Usually, the failure is used to report failures directly related to
what this code is testing. If /proc is unavailable, it's an
environment setup issue, which is more fitting for SKIP, otherwise, we
wouldn't need "SKIP" - we'd just report all environment requirements
checked as failures.

Unless you mean that "/proc" is always available and can never be
unavailable in any selftest environment? Then, I can change to use the
failure reporting.

> > +};
> > +
> > +FIXTURE_TEARDOWN(basic)
> > +{
> > +     if (self->maps)
> > +             fclose(self->maps);
> > +};
> > +
> > +FIXTURE_VARIANT(basic)
> > +{
> > +     char *name;
> > +};
> > +
> > +FIXTURE_VARIANT_ADD(basic, vdso) {
> > +     .name = VDSO_NAME,
> > +};
> > +
> > +FIXTURE_VARIANT_ADD(basic, vvar) {
> > +     .name = VVAR_NAME,
> > +};
> > +
> > +FIXTURE_VARIANT_ADD(basic, vvar_vclock) {
> > +     .name = VVAR_VCLOCK_NAME,
> > +};
> > +
> > +FIXTURE_VARIANT_ADD(basic, sigpage) {
> > +     .name = SIGPAGE_NAME,
> > +};
> > +
> > +FIXTURE_VARIANT_ADD(basic, vectors) {
> > +     .name = VECTORS_NAME,
> > +};
> > +
> > +FIXTURE_VARIANT_ADD(basic, uprobes) {
> > +     .name = UPROBES_NAME,
> > +};
>
> I love seeing variants used in the test harness. :)
>
Ya, copied from landlock selftest :-)

> > +
> > +TEST_F(basic, is_sealed)
> > +{
> > +     if (!has_mapping(variant->name, self->maps)) {
> > +             SKIP(return, "could not found the mapping, %s",
>
> typo nit: "find" instead of "found"
>
> > +                     variant->name);
> > +     }
> > +
> > +     EXPECT_TRUE(mapping_is_sealed(variant->name, self->maps));
> > +};
>
> This is a good "positive" test, but I'd like to see a negative test
> added as well. (This adds robustness against something going "all wrong"
> or "all right", like imagine that someone adds a VmFlags string named
> "slow", suddenly this test will always pass due to matching "sl". With
> a negative test added, it will fail when it finds "sl" when it's not
> expected.) For example, also check "[stack]" and "[heap]" and expect
> them NOT to be sealed.
>
> You could update the variant as:
>
> FIXTURE_VARIANT(basic)
> {
>         char *name;
>         bool sealed;
> };
>
> FIXTURE_VARIANT_ADD(basic, vdso) {
>         .name = "[vdso]",
>         .sealed = true,
> };
>
> FIXTURE_VARIANT_ADD(basic, stack) {
>         .name = "[stack]",
>         .sealed = false,
> };
>
> And then update the is_sealed test to:
>
>         EXPECT_EQ(variant->sealed, mapping_is_sealed(variant->name, self->maps));
>
The challenge is that I'm unsure how to detect
"CONFIG_MSEAL_SYSTEM_MAPPINGS" from selftest runtime.

Without that, the test can't reliably set the "sealed" flag. Lorenzo
suggested parsing /proc/config.gz, but I responded, "None of the
existing selftests use this pattern, and I'm not sure /proc/config.gz
is enabled in the default kernel config." [1].

To work around this, in this version, I add
selftests/mseal_system_mappings/config to indicate
CONFIG_MSEAL_SYSTEM_MAPPINGS=y is a mandatory requirement for running
this test. Therefore, this selftest assumes the ".sealed" is always
true, i.e. no negative test.

I'm looking to Linux Kernel Self-Test (LKST) and Shuah Khan for
guidance/suggestion on handling different kernel config variants
within selftest.

Thanks
-Jeff
[1] https://lore.kernel.org/all/60f5b979-2969-4cb0-ad3d-262908869c5f@lucifer.local/

> --
> Kees Cook