Re: [RFC PATCH v5 2/7] selftests: x86: test_mremap_vdso: skip if vdso is msealed

[Kees Cook <kees@kernel.org>]

On February 13, 2025 2:20:01 PM PST, Jeff Xu <jeffxu@chromium.org> wrote:
>On Thu, Feb 13, 2025 at 11:28 AM Kees Cook <kees@kernel.org> wrote:
>>
>>
>>
>> On February 13, 2025 6:14:00 AM PST, Jeff Xu <jeffxu@chromium.org> wrote:
>> >On Wed, Feb 12, 2025 at 5:04 AM Thomas Weißschuh
>> ><thomas.weissschuh@linutronix.de> wrote:
>> >>
>> >> On Wed, Feb 12, 2025 at 03:21:50AM +0000, jeffxu@chromium.org wrote:
>> >> > From: Jeff Xu <jeffxu@chromium.org>
>> >> >
>> >> > Add code to detect if the vdso is memory sealed, skip the test
>> >> > if it is.
>> >> >
>> >> > Signed-off-by: Jeff Xu <jeffxu@chromium.org>
>> >> > ---
>> >> >  .../testing/selftests/x86/test_mremap_vdso.c  | 38 +++++++++++++++++++
>> >> >  1 file changed, 38 insertions(+)
>> >> >
>> >> > diff --git a/tools/testing/selftests/x86/test_mremap_vdso.c b/tools/testing/selftests/x86/test_mremap_vdso.c
>> >> > index d53959e03593..c68077c56b22 100644
>> >> > --- a/tools/testing/selftests/x86/test_mremap_vdso.c
>> >> > +++ b/tools/testing/selftests/x86/test_mremap_vdso.c
>> >> > @@ -14,6 +14,7 @@
>> >> >  #include <errno.h>
>> >> >  #include <unistd.h>
>> >> >  #include <string.h>
>> >> > +#include <stdbool.h>
>> >> >
>> >> >  #include <sys/mman.h>
>> >> >  #include <sys/auxv.h>
>> >> > @@ -55,13 +56,50 @@ static int try_to_remap(void *vdso_addr, unsigned long size)
>> >> >
>> >> >  }
>> >> >
>> >> > +#define VDSO_NAME "[vdso]"
>> >> > +#define VMFLAGS "VmFlags:"
>> >> > +#define MSEAL_FLAGS "sl"
>> >> > +#define MAX_LINE_LEN 512
>> >> > +
>> >> > +bool vdso_sealed(FILE *maps)
>> >> > +{
>> >> > +     char line[MAX_LINE_LEN];
>> >> > +     bool has_vdso = false;
>> >> > +
>> >> > +     while (fgets(line, sizeof(line), maps)) {
>> >> > +             if (strstr(line, VDSO_NAME))
>> >> > +                     has_vdso = true;
>> >> > +
>> >> > +             if (has_vdso && !strncmp(line, VMFLAGS, strlen(VMFLAGS))) {
>> >> > +                     if (strstr(line, MSEAL_FLAGS))
>> >> > +                             return true;
>> >> > +
>> >> > +                     return false;
>> >>
>> >> This only tests that any mapping after the vdso is sealed.
>> >
>> >The code above begins by searching for the "[vdso]" string, then looks
>> >for the first line that starts with "VmFlags:", and looks for the "sl"
>> >substring within that line. We're assuming the format of smaps won't
>> >change from release to release.
>>
>> Oh, I missed this in my reviews: nothing _resets_ has_vdso to false, so if any other mapping follows vdso that happens to be sealed, this will return true...
>>
>It won't return the next mapping's sealing flag.
>After finding the "[vdso]", if the next line that contains VMFLAGS
>doesn't have the "sl" flag, the function returns false immediately.

Oh! Agh, yes. You are right, this is all fine.

>I can switch to vm_util, I will need to add a new parsing function in
>vm_util, the existing __get_smap_entry() only searches for vm's
>starting address, not name.

Unless someone feels strongly about this, my instinct is to avoid the higher complexity of a cross-test thing.

-Kees


-- 
Kees Cook