Re: [PATCH] block: Check ADMIN before NICE for IOPRIO_CLASS_RT

Alistair Delva [off-list ref] writes:

On Mon, Nov 15, 2021 at 11:04 AM Ondrej Mosnacek [off-list ref] wrote:

quoted

On Mon, Nov 15, 2021 at 7:14 PM Alistair Delva [off-list ref] wrote:

quoted

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.

quoted

quoted

Fixes: 9d3a39a5f1e4 ("block: grant IOPRIO_CLASS_RT to CAP_SYS_NICE")
Signed-off-by: Alistair Delva <redacted>
Cc: Khazhismel Kumykov <redacted>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: selinux@vger.kernel.org
Cc: linux-security-module@vger.kernel.org
Cc: kernel-team@android.com
Cc: stable@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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