apparmor: don't try to replace stale label in ptrace access check

As a comment above begin_current_label_crit_section() explains,
begin_current_label_crit_section() must run in sleepable context because
when label_is_stale() is true, aa_replace_current_label() runs, which uses
prepare_creds(), which can sleep.
Until now, the ptrace access check (which runs with a task lock held)
violated this rule.

Also add a might_sleep() assertion to begin_current_label_crit_section(),
because asserts are less likely to be ignored than comments.

Fixes: b2d09ae449ced ("apparmor: move ptrace checks to using labels")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

2 files changed, 4 insertions, 2 deletions

diff --git a/security/apparmor/include/cred.h b/security/apparmor/include/cred.h
index e287b7d0d4be..265ae6641a06 100644
--- a/security/apparmor/include/cred.h
+++ b/security/apparmor/include/cred.h
@@ -151,6 +151,8 @@ static inline struct aa_label *begin_current_label_crit_section(void)
 {
         struct aa_label *label = aa_current_raw_label();
 
+        might_sleep();
+
         if (label_is_stale(label)) {
                 label = aa_get_newest_label(label);
                 if (aa_replace_current_label(label) == 0)
diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 74f17376202b..f09fea0b4db7 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -114,13 +114,13 @@ static int apparmor_ptrace_access_check(struct task_struct *child,
         struct aa_label *tracer, *tracee;
         int error;
 
-        tracer = begin_current_label_crit_section();
+        tracer = __begin_current_label_crit_section();
         tracee = aa_get_task_label(child);
         error = aa_may_ptrace(tracer, tracee,
                         (mode & PTRACE_MODE_READ) ? AA_PTRACE_READ
                                                   : AA_PTRACE_TRACE);
         aa_put_label(tracee);
-        end_current_label_crit_section(tracer);
+        __end_current_label_crit_section(tracer);
 
         return error;
 }