ptrace: do not audit capability check when outputing /proc/pid/stat

Reading /proc/pid/stat of another process checks if one has ptrace permissions
on that process.  If one does have permissions it outputs some data about the
process which might have security and attack implications.  If the current
task does not have ptrace permissions the read still works, but those fields
are filled with inocuous (0) values.  Since this check and a subsequent denial
is not a violation of the security policy we should not audit such denials.

This can be quite useful to removing ptrace broadly across a system without
flooding the logs when ps is run or something which harmlessly walks proc.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Serge E. Hallyn <serge.hallyn@canonical.com>

4 files changed, 15 insertions, 6 deletions

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 3a1dafd228d1..ddffd7a88b97 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -380,7 +380,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 
         state = *get_task_state(task);
         vsize = eip = esp = 0;
-        permitted = ptrace_may_access(task, PTRACE_MODE_READ);
+        permitted = ptrace_may_access(task, PTRACE_MODE_READ | PTRACE_MODE_NOAUDIT);
         mm = get_task_mm(task);
         if (mm) {
                 vsize = task_vsize(mm);
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 800f113bea66..a27e56ca41a4 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -127,8 +127,9 @@ extern void __ptrace_link(struct task_struct *child,
                           struct task_struct *new_parent);
 extern void __ptrace_unlink(struct task_struct *child);
 extern void exit_ptrace(struct task_struct *tracer);
-#define PTRACE_MODE_READ   1
-#define PTRACE_MODE_ATTACH 2
+#define PTRACE_MODE_READ        0x01
+#define PTRACE_MODE_ATTACH      0x02
+#define PTRACE_MODE_NOAUDIT     0x04
 /* Returns 0 on success, -errno on denial. */
 extern int __ptrace_may_access(struct task_struct *task, unsigned int mode);
 /* Returns true on success, false on denial. */
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 210bbf045ee9..c890ac9a7962 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -161,6 +161,14 @@ int ptrace_check_attach(struct task_struct *child, bool ignore_state)
         return ret;
 }
 
+static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
+{
+        if (mode & PTRACE_MODE_NOAUDIT)
+                return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
+        else
+                return has_ns_capability(current, ns, CAP_SYS_PTRACE);
+}
+
 int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 {
         const struct cred *cred = current_cred(), *tcred;
@@ -187,7 +195,7 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
              cred->gid == tcred->sgid &&
              cred->gid == tcred->gid))
                 goto ok;
-        if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
+        if (ptrace_has_cap(tcred->user->user_ns, mode))
                 goto ok;
         rcu_read_unlock();
         return -EPERM;
@@ -196,7 +204,7 @@ ok:
         smp_rmb();
         if (task->mm)
                 dumpable = get_dumpable(task->mm);
-        if (!dumpable && !ns_capable(task_user_ns(task), CAP_SYS_PTRACE))
+        if (!dumpable  && !ptrace_has_cap(task_user_ns(task), mode))
                 return -EPERM;
 
         return security_ptrace_access_check(task, mode);
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index c9605c4a2e08..14f94cd29c80 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -1809,7 +1809,7 @@ static int selinux_ptrace_access_check(struct task_struct *child,
         if (rc)
                 return rc;
 
-        if (mode == PTRACE_MODE_READ) {
+        if (mode & PTRACE_MODE_READ) {
                 u32 sid = current_sid();
                 u32 csid = task_sid(child);
                 return avc_has_perm(sid, csid, SECCLASS_FILE, FILE__READ, NULL);