linux-kernel - Re: selftests/capabilities: test FAIL on linux mainline and linux-next and PASS on linux-4.4.70+

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Date:   Thu, 29 Jun 2017 09:23:15 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuahkh@....samsung.com>, Greg KH <greg@...ah.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: selftests/capabilities: test FAIL on linux mainline and linux-next and PASS on linux-4.4.70+

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Andy Lutomirski <luto@...nel.org> writes:
>>
>> Hi Eric-
>>
>> This is rather odd.  The selftest
>> (tools/testing/selftests/capabilities/test_execve), run as root, fails
>> on current kernels.  The failure is worked around by this:
>>
>> diff --git a/tools/testing/selftests/capabilities/test_execve.c
>> b/tools/testing/selftests/capabilities/test_execve.c
>> index 10a21a958aaf..6db60889b211 100644
>> --- a/tools/testing/selftests/capabilities/test_execve.c
>> +++ b/tools/testing/selftests/capabilities/test_execve.c
>> @@ -139,8 +139,8 @@ static void chdir_to_tmpfs(void)
>>         if (chdir(cwd) != 0)
>>                 err(1, "chdir to private tmpfs");
>>
>> -       if (umount2(".", MNT_DETACH) != 0)
>> -               err(1, "detach private tmpfs");
>> +//     if (umount2(".", MNT_DETACH) != 0)
>> +//             err(1, "detach private tmpfs");
>>  }
>>
>>  static void copy_fromat_to(int fromfd, const char *fromname, const
>> char *toname)
>>
>> I think this is due to the line:
>>
>> p->mnt_ns = NULL;
>>
>> in umount_tree().  The test is putting us into a situation in which
>> our cwd has ->mnt_ns = NULL, which is making it act as if it's nosuid.
>> I can imagine this breaking some weird user code (like my test!).  Is
>> it a real problem, though?

I just wanted to follow up and say this the mnt_may_suid test appears
to be doing exactly what it was designed to do.

It's goal is not to allow a suid exec from another mount namespace and
in this test the umount2(".", MNT_DETACH) creates a poor man's mount
namespace.

So assuming that we want to not allow execing executables from other
mount namespaces the behavior appears to be exactly correct in this
case.

Eric