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

[Jeff Xu]

On Thu, Feb 13, 2025 at 6:52 PM Kees Cook <kees@xxxxxxxxxx> wrote:
>
>
>
> On February 13, 2025 2:20:01 PM PST, Jeff Xu <jeffxu@xxxxxxxxxxxx> wrote:
> >On Thu, Feb 13, 2025 at 11:28 AM Kees Cook <kees@xxxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On February 13, 2025 6:14:00 AM PST, Jeff Xu <jeffxu@xxxxxxxxxxxx> wrote:
> >> >On Wed, Feb 12, 2025 at 5:04 AM Thomas Weißschuh
> >> ><thomas.weissschuh@xxxxxxxxxxxxx> wrote:
> >> >>
> >> >> On Wed, Feb 12, 2025 at 03:21:50AM +0000, jeffxu@xxxxxxxxxxxx wrote:
> >> >> > From: Jeff Xu <jeffxu@xxxxxxxxxxxx>
> >> >> >
> >> >> > Add code to detect if the vdso is memory sealed, skip the test
> >> >> > if it is.
> >> >> >
> >> >> > Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx>
> >> >> > ---
> >> >> >  .../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.
>
OK. I will keep the existing test.

If we decide to use vm_util, it would be best to refactor it
separately later on. The existing vm_util can't be used as is for my
needs, so some refactoring would be necessary.


Thanks
-Jeff

> -Kees
>
>
> --
> Kees Cook