On Mon, Nov 15, 2021 at 01:42:54PM -0800, Casey Schaufler wrote:

On 11/15/2021 11:08 AM, Alistair Delva wrote:

quoted

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.

Because SELinux policy is configuration data, not system code.
One is free to modify SELinux policy to suit one's whims without
making any change to the Linux kernel or its APIs.

Sure, but the problem here is when the kernel is updated and the
userspace configuration is not changed and then the kernel can not boot
or has other problems.  That is a kernel regression.

quoted

  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.

This is an artifact of separating policy from mechanism. The
capability mechanism does not suffer from this issue because
it embodies its policy. SELinux, Smack, AppArmor and "containers"
are vulnerable to it because they explicitly deny the kernel and
kernel developers permission to make assumptions about how they
define "policy".

quoted

  So, we have to touch
userspace. If this is just how things are, I can certainly update our
init.te definitions.

If SELinux was a required kernel mechanism and the policy was
included in the kernel tree you might be able to argue that
kernel developers are responsible for changes to SELinux policy.
But it ain't, and it isn't.   By design.

Again, when you change the logic in the kernel that then requires you to
also somehow change userspace files in order to keep the kernel booting
properly, that is a problem.

Same thing if we changed the tty api to require different options to be
handled differently.  There is nothing "special" about security policies
from any other user/kernel interaction and api.

thanks,

greg k-h