It seems like the CAP_SETUID_RANGE idea proposed by Serge is mainly a
way to avoid silently breaking programs that run with CAP_SETUID,
which could cause security vulnerabilities. Serge, does Casey's
suggestion (killing processes that try to perform unapproved
transitions) make sense to you as an alternate way to safeguard
against this? Sure there could be regressions, but they would only
happen to users for which whitelist policies had been configured, and
killing processes should be an effective way of identifying any
missing whitelist policies on the system for some restricted user. One
less attractive thing about adding a CAP_SETUID_RANGE capability would
be that more of the common kernel code would have to be modified to
add a new capability, whereas currently the LSM just uses the LSM
hooks.

The other unresolved aspect here (discussed by Stephen above) is how
to "know" that ns_capable(..., CAP_SETUID) was called by sys_set*uid
and not some other kernel code path. The reason we need to know this
is to be able to distinguish id transitions from other privileged
actions (e.g. create/modify/enter user namespace). Certain transitions
should be allowed for whitelisted users, but the other privileged
actions should be denied (or else the security hardening provided by
this LSM is significantly weakened). Do people think the current
reliance on comparing the return value of syscall_get_nr() to
arch-specific syscall constants (e.g. __NR_setuid) is a deal-breaker
and we should find an arch-independent solution such as the one
proposed by Stephen? Or is checking against arch-specific constants
not a big deal and the code can stay as is?
On Fri, Nov 2, 2018 at 12:22 PM Serge E. Hallyn [off-list ref] wrote:

Quoting Casey Schaufler (casey@schaufler-ca.com):

quoted

On 11/2/2018 11:30 AM, Serge E. Hallyn wrote:

quoted

Quoting Casey Schaufler (casey@schaufler-ca.com):

quoted

Let me suggest a change to the way your LSM works
that would reduce my concerns. Rather than refusing to
make a UID change that isn't on your whitelist, kill a
process that makes a prohibited request. This mitigates
the problem where a process doesn't check for an error
return. Sure, your system will be harder to get running
until your whitelist is complete, but you'll avoid a
whole category of security bugs.

Might also consider not restricting CAP_SETUID, but instead adding a
new CAP_SETUID_RANGE capability.  That way you can be sure there will be
no regressions with any programs which run with CAP_SETUID.

Though that violates what Casey was just arguing halfway up the email.

I know that it's hard to believe 20 years after the fact,
but the POSIX group worked very hard to ensure that the granularity
of capabilities was correct for the security policy that the
interfaces defined in P1003.1. What would CAP_SETUID_RANGE mean?

CAP_SETUID would mean you can switch to any uid.

CAP_SETUID_RANGE would mean you could make the transitions which have
been defined through <handwave> some mechanism.  Be it prctl, or some
new security.uidrange xattr, or the mechanism Micah proposed, except
it only applies for CAP_SETUID_RANGE not CAP_SETUID.

-serge