[PATCH] block: Check ADMIN before NICE for IOPRIO_CLASS_RT

[Dominick Grift]

Alistair Delva <adelva at google.com> writes:

> On Mon, Nov 15, 2021 at 11:04 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>>
>> On Mon, Nov 15, 2021 at 7:14 PM Alistair Delva <adelva at google.com> wrote:
>> > Booting to Android userspace on 5.14 or newer triggers the following
>> > SELinux denial:
>> >
>> > avc: denied { sys_nice } for comm="init" capability=23
>> >      scontext=u:r:init:s0 tcontext=u:r:init:s0 tclass=capability
>> >      permissive=0
>> >
>> > Init is PID 0 running as root, so it already has CAP_SYS_ADMIN. For
>> > better compatibility with older SEPolicy, check ADMIN before NICE.
>>
>> But with this patch you in turn punish the new/better policies that
>> try to avoid giving domains CAP_SYS_ADMIN unless necessary (using only
>> the more granular capabilities wherever possible), which may now get a
>> bogus sys_admin denial. IMHO the order is better as it is, as it
>> motivates the "good" policy writing behavior - i.e. spelling out the
>> capability permissions more explicitly and avoiding CAP_SYS_ADMIN.
>>
>> IOW, if you domain does CAP_SYS_NICE things, and you didn't explicitly
>> grant it that (and instead rely on the CAP_SYS_ADMIN fallback), then
>> the denial correctly flags it as an issue in your policy and
>> encourages you to add that sys_nice permission to the domain. Then
>> when one beautiful hypothetical day the CAP_SYS_ADMIN fallback is
>> removed, your policy will be ready for that and things will keep
>> working.
>>
>> Feel free to carry that patch downstream if patching the kernel is
>> easier for you than fixing the policy, but for the upstream kernel
>> this is just a step in the wrong direction.
>
> I'm personally fine with this position, but I am curious why "never
> break userspace" doesn't apply to SELinux policies. At the end of the
> day, booting 5.13 or older, we don't get a denial, and there's nothing
> for the sysadmin to do. On 5.14 and newer, we get denials. This is a
> common pattern we see each year: some new capability or permission is
> required where it wasn't required before, and there's no compatibility
> mechanism to grandfather in old policies. So, we have to touch
> userspace. If this is just how things are, I can certainly update our
> init.te definitions.

User space is not broken? If you just ignore this AVC denial then it
will pass on cap_sys_admin. In other words everything still works, you
only get a AVC denial for cap_sys_nice now.

>
>> > Fixes: 9d3a39a5f1e4 ("block: grant IOPRIO_CLASS_RT to CAP_SYS_NICE")
>> > Signed-off-by: Alistair Delva <adelva at google.com>
>> > Cc: Khazhismel Kumykov <khazhy at google.com>
>> > Cc: Bart Van Assche <bvanassche at acm.org>
>> > Cc: Serge Hallyn <serge at hallyn.com>
>> > Cc: Jens Axboe <axboe at kernel.dk>
>> > Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
>> > Cc: Paul Moore <paul at paul-moore.com>
>> > Cc: selinux at vger.kernel.org
>> > Cc: linux-security-module at vger.kernel.org
>> > Cc: kernel-team at android.com
>> > Cc: stable at vger.kernel.org # v5.14+
>> > ---
>> >  block/ioprio.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/block/ioprio.c b/block/ioprio.c
>> > index 0e4ff245f2bf..4d59c559e057 100644
>> > --- a/block/ioprio.c
>> > +++ b/block/ioprio.c
>> > @@ -69,7 +69,7 @@ int ioprio_check_cap(int ioprio)
>> >
>> >         switch (class) {
>> >                 case IOPRIO_CLASS_RT:
>> > -                       if (!capable(CAP_SYS_NICE) && !capable(CAP_SYS_ADMIN))
>> > +                       if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
>> >                                 return -EPERM;
>> >                         fallthrough;
>> >                         /* rt has prio field too */
>> > --
>> > 2.34.0.rc1.387.gb447b232ab-goog
>> >
>>
>> --
>> Ondrej Mosnacek
>> Software Engineer, Linux Security - SELinux kernel
>> Red Hat, Inc.
>>

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Dominick Grift