[PATCH] Audit: close race between audit_syscall_exit and proc_loginuid_read

[Eric Paris <eparis@redhat.com>]

audit_syscall_exit() calls audit_get_context() which returns the
tsk->audit_context but then also sets tsk->audit_context=NULL.  A few
lines later audit_syscall_exit sets the audit_context (either back to
audit_context or to a different context)

During this window when tsk->audit_context is NULL it is possible that
another process will try to read /proc/pid/loginuid and will get a -1.
There does not appear to be a good reason to set audit_context to null
in the get function so this patch merely leaves the audit_context alone
so there is no period of time in which audit_context is not pointing to
a valid context.

Signed-off-by: Eric Paris <eparis@redhat.com>

---

Easily tested, I opened 2 shells.  

shell #1 I read /proc/shell2/loginuid in a very very tight loop.
shell #2 I ran a tight loop adding and removing audit rules

without the patch I often saw -1 as the loginuid but with the patch I
always got the correct behavior.


 kernel/auditsc.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 04f3ffb..62ace9b 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -598,7 +598,6 @@ static inline struct audit_context *audit_get_context(struct task_struct *tsk,
 
 get_context:
 
-	tsk->audit_context = NULL;
 	return context;
 }
 
@@ -1135,6 +1134,8 @@ void audit_free(struct task_struct *tsk)
 	if (likely(!context))
 		return;
 
+	tsk->audit_context = NULL;
+
 	/* Check for system calls that do not go through the exit
 	 * function (e.g., exit_group), then free context block. 
 	 * We use GFP_ATOMIC here because we might be doing this 
@@ -1266,9 +1267,9 @@ void audit_syscall_exit(int valid, long return_code)
 
 	if (context->previous) {
 		struct audit_context *new_context = context->previous;
+		tsk->audit_context = new_context;
 		context->previous  = NULL;
 		audit_free_context(context);
-		tsk->audit_context = new_context;
 	} else {
 		audit_free_names(context);
 		audit_free_aux(context);