[RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC_RESTRICT securebits

Wed Jul 10 09:58:25 UTC 2024

On Tue, Jul 09, 2024 at 02:57:43PM -0700, Jeff Xu wrote:
> On Tue, Jul 9, 2024 at 1:42 PM Mickaël Salaün <mic at digikod.net> wrote:
> >
> > On Mon, Jul 08, 2024 at 03:07:24PM -0700, Jeff Xu wrote:
> > > On Mon, Jul 8, 2024 at 2:25 PM Steve Dower <steve.dower at python.org> wrote:
> > > >
> > > > On 08/07/2024 22:15, Jeff Xu wrote:
> > > > > IIUC:
> > > > > CHECK=0, RESTRICT=0: do nothing, current behavior
> > > > > CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results.
> > > > > CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception.
> > > > > CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except
> > > > > those in the "checked-and-allowed" list.
> > > >
> > > > I had much the same question for Mickaël while working on this.
> > > >
> > > > Essentially, "CHECK=0, RESTRICT=1" means to restrict without checking.
> > > > In the context of a script or macro interpreter, this just means it will
> > > > never interpret any scripts. Non-binary code execution is fully disabled
> > > > in any part of the process that respects these bits.
> > > >
> > > I see, so Mickaël does mean this will block all scripts.
> >
> > That is the initial idea.
> >
> > > I guess, in the context of dynamic linker, this means: no more .so
> > > loading, even "dlopen" is called by an app ?  But this will make the
> > > execve()  fail.
> >
> > Hmm, I'm not sure this "CHECK=0, RESTRICT=1" configuration would make
> > sense for a dynamic linker except maybe if we want to only allow static
> > binaries?
> >
> > The CHECK and RESTRICT securebits are designed to make it possible a
> > "permissive mode" and an enforcement mode with the related locked
> > securebits.  This is why this "CHECK=0, RESTRICT=1" combination looks a
> > bit weird.  We can replace these securebits with others but I didn't
> > find a better (and simple) option.  I don't think this is an issue
> > because with any security policy we can create unusable combinations.
> > The three other combinations makes a lot of sense though.
> >
> If we need only handle 3  combinations,  I would think something like
> below is easier to understand, and don't have wield state like
> CHECK=0, RESTRICT=1

The "CHECK=0, RESTRICT=1" is useful for script interpreter instances
that should not interpret any command from users e.g., but only execute
script files.

> 
> XX_RESTRICT: when true: Perform the AT_CHECK, and deny the executable
> after AT_CHECK fails.

> XX_RESTRICT_PERMISSIVE:  take effect when XX_RESTRICT is true. True
> means Ignoring the AT_CHECK result.

We get a similar weird state with XX_RESTRICT_PERMISSIVE=1 and
XX_RESTRICT=0

As a side note, for compatibility reasons, by default all securebits
must be 0, and this must translate to no restriction.

> 
> Or
> 
> XX_CHECK: when true: Perform the AT_CHECK.
> XX_CHECK_ENFORCE takes effect only when XX_CHECK is true.   True means
> restrict the executable when AT_CHECK failed; false means ignore the
> AT_CHECK failure.

We get a similar weird state with XX_CHECK_ENFORCE=1 and XX_CHECK=0

> 
> Of course, we can replace XX_CHECK_ENFORCE with XX_RESTRICT.
> Personally I think having _CHECK_ in the name implies the XX_CHECK
> needs to be true as a prerequisite for this flag , but that is my
> opinion only. As long as the semantics are clear as part of the
> comments of definition in code,  it is fine.

Here is another proposal:

We can change a bit the semantic by making it the norm to always check
file executability with AT_CHECK, and using the securebits to restrict
file interpretation and/or command injection (e.g. user supplied shell
commands).  Non-executable checked files can be reported/logged at the
kernel level, with audit, configured by sysadmins.

New securebits (feel free to propose better names):

- SECBIT_EXEC_RESTRICT_FILE: requires AT_CHECK to pass.

- SECBIT_EXEC_DENY_INTERACTIVE: deny any command injection via
  command line arguments, environment variables, or configuration files.
  This should be ignored by dynamic linkers.  We could also have an
  allow-list of shells for which this bit is not set, managed by an
  LSM's policy, if the native securebits scoping approach is not enough.

Different modes for script interpreters:

1. RESTRICT_FILE=0 DENY_INTERACTIVE=0 (default)
   Always interpret scripts, and allow arbitrary user commands.
   => No threat, everyone and everything is trusted, but we can get
   ahead of potential issues with logs to prepare for a migration to a
   restrictive mode.

2. RESTRICT_FILE=1 DENY_INTERACTIVE=0
   Deny script interpretation if they are not executable, and allow
   arbitrary user commands.
   => Threat: (potential) malicious scripts run by trusted (and not
      fooled) users.  That could protect against unintended script
      executions (e.g. sh /tmp/*.sh).
   ==> Makes sense for (semi-restricted) user sessions.

3. RESTRICT_FILE=1 DENY_INTERACTIVE=1
   Deny script interpretation if they are not executable, and also deny
   any arbitrary user commands.
   => Threat: malicious scripts run by untrusted users.
   ==> Makes sense for system services executing scripts.

4. RESTRICT_FILE=0 DENY_INTERACTIVE=1
   Always interpret scripts, but deny arbitrary user commands.
   => Goal: monitor/measure/assess script content (e.g. with IMA/EVM) in
      a system where the access rights are not (yet) ready.  Arbitrary
      user commands would be much more difficult to monitor.
   ==> First step of restricting system services that should not
       directly pass arbitrary commands to shells.

> 
> Thanks
> -Jeff
> 
> 
> > >
> > > > "CHECK=1, RESTRICT=1" means to restrict unless AT_CHECK passes. This
> > > > case is the allow list (or whatever mechanism is being used to determine
> > > > the result of an AT_CHECK check). The actual mechanism isn't the
> > > > business of the script interpreter at all, it just has to refuse to
> > > > execute anything that doesn't pass the check. So a generic interpreter
> > > > can implement a generic mechanism and leave the specifics to whoever
> > > > configures the machine.
> > > >
> > > In the context of dynamic linker. this means:
> > > if .so passed the AT_CHECK, ldopen() can still load it.
> > > If .so fails the AT_CHECK, ldopen() will fail too.
> >
> > Correct
> >
> > >
> > > Thanks
> > > -Jeff
> > >
> > > > The other two case are more obvious. "CHECK=0, RESTRICT=0" is the
> > > > zero-overhead case, while "CHECK=1, RESTRICT=0" might log, warn, or
> > > > otherwise audit the result of the check, but it won't restrict execution.
> > > >
> > > > Cheers,
> > > > Steve