-----Original Message-----
From: Stephen Smalley [mailto:sds at tycho.nsa.gov]
Sent: Thursday, September 27, 2018 8:50 AM
To: Schaufler, Casey <redacted>; kernel-
hardening at lists.openwall.com; linux-kernel at vger.kernel.org; linux-security-
module at vger.kernel.org; selinux at tycho.nsa.gov; Hansen, Dave
[off-list ref]; Dock, Deneen T [off-list ref];
kristen at linux.intel.com; arjan at linux.intel.com; Paul Moore <paul@paul-
moore.com>
Subject: Re: [PATCH v5 3/5] SELinux: Prepare for PTRACE_MODE_SCHED

On 09/26/2018 04:34 PM, Casey Schaufler wrote:

quoted

From: Casey Schaufler <casey@schaufler-ca.com>

A ptrace access check with mode PTRACE_MODE_SCHED gets called
from process switching code. This precludes the use of audit or avc,
as the locking is incompatible. The only available check that
can be made without using avc is a comparison of the secids.
This is not very satisfactory as it will indicate possible
vulnerabilies much too aggressively.

Can you document (in the patch description and/or in the inline
documentation in lsm_hooks.h) what locks can be safely used when this
hook is called with PTRACE_MODE_SCHED?  rcu_read_lock() seemingly must
be safe since it is being called by task_sid() below. Are any other
locking primitives safe?

Peter Zijlstra [off-list ref] had this comment on
locking in the SELinux ptrace path. 

 	avc_has_perm_noaudit()
	  security_compute_av()
	    read_lock(&state->ss->policy_rwlock);
	  avc_insert()
	    spin_lock_irqsave();
	  avc_denied()
	    avc_update_node()
	      spin_lock_irqsave();

under the scheduler's raw_spinlock_t, which are invalid lock nestings.

I don't know that it would be impossible to address these issues,
but as many people have noted over the years I am not now
nor have ever been an expert on locking.

Does the PTRACE_MODE_SCHED check have to occur while holding the
scheduler lock, or could it be performed before taking the lock?

My understanding is that the lock is required.

 

Can you cite the commit or patch posting (e.g. from lore or patchwork)
that defines PTRACE_MODE_SCHED and its usage as part of the patch
description for context?  Is this based on the v7 patchset, e.g.
https://lore.kernel.org/lkml/nycvar.YFH.7.76.1809251437340.15880 at cbobk.fhf
r.pm/

Yes, that's the one. Sorry, I should have identified that.

quoted

Signed-off-by: Casey Schaufler <redacted>
---
  security/selinux/hooks.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index ad9a9b8e9979..160239791007 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c

@@ -2267,6 +2267,8 @@ static int selinux_ptrace_access_check(struct

task_struct *child,

quoted

  	u32 sid = current_sid();
  	u32 csid = task_sid(child);

+	if (mode & PTRACE_MODE_SCHED)
+		return sid == csid ? 0 : -EACCES;

IIUC, this logic is essentially the same as the uid-based check,
including the fact that even a "privileged" process is not given any
special handling since they always return false from ptrace_has_cap()
for PTRACE_MODE_SCHED. If they are ok with applying IBPB whenever uids
differ, then doing so whenever sids/contexts differ does not seem like
an onerous thing.

quoted

  	if (mode & PTRACE_MODE_READ)
  		return avc_has_perm(&selinux_state,
  				    sid, csid, SECCLASS_FILE, FILE__READ,

NULL);

quoted