From: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>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?
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.
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.
Signed-off-by: Casey Schaufler <casey.schaufler@xxxxxxxxx>
---
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;
if (mode & PTRACE_MODE_READ)
return avc_has_perm(&selinux_state,
sid, csid, SECCLASS_FILE, FILE__READ, NULL);