| Message ID | 20180926203446.2004-4-casey.schaufler@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | LSM: Support ptrace sidechannel access checks | expand |
On 09/26/2018 04:34 PM, Casey Schaufler wrote: > 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? Does the PTRACE_MODE_SCHED check have to occur while holding the scheduler lock, or could it be performed before taking the lock? 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@cbobk.fhfr.pm/ > > Signed-off-by: Casey Schaufler <casey.schaufler@intel.com> > --- > 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, > 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. > if (mode & PTRACE_MODE_READ) > return avc_has_perm(&selinux_state, > sid, csid, SECCLASS_FILE, FILE__READ, NULL); >
> -----Original Message----- > From: Stephen Smalley [mailto:sds@tycho.nsa.gov] > Sent: Thursday, September 27, 2018 8:50 AM > To: Schaufler, Casey <casey.schaufler@intel.com>; kernel- > hardening@lists.openwall.com; linux-kernel@vger.kernel.org; linux-security- > module@vger.kernel.org; selinux@tycho.nsa.gov; Hansen, Dave > <dave.hansen@intel.com>; Dock, Deneen T <deneen.t.dock@intel.com>; > kristen@linux.intel.com; arjan@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: > > 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 <peterz@infradead.org> 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@cbobk.fhf > r.pm/ Yes, that's the one. Sorry, I should have identified that. > > > > > Signed-off-by: Casey Schaufler <casey.schaufler@intel.com> > > --- > > 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, > > 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. > > > > if (mode & PTRACE_MODE_READ) > > return avc_has_perm(&selinux_state, > > sid, csid, SECCLASS_FILE, FILE__READ, > NULL); > >
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, u32 sid = current_sid(); u32 csid = task_sid(child); + if (mode & PTRACE_MODE_SCHED) + return sid == csid ? 0 : -EACCES; if (mode & PTRACE_MODE_READ) return avc_has_perm(&selinux_state, sid, csid, SECCLASS_FILE, FILE__READ, NULL);