[PATCH] lsm: get comm using lock to avoid race in string printing

[Richard Guy Briggs]

When task->comm is passed directly to audit_log_untrustedstring() without
getting a copy or using the task_lock, there is a race that could happen that
would output a NULL (\0) in the output string that would effectively truncate
the rest of the report text after the comm= field in the audit, losing fields.

Use get_task_comm() to get a copy while acquiring the task_lock to prevent
this and to prevent the result from being a mixture of old and new values of
comm.

Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
---
I've manually checked for locking issues and found none.  I've also enabled all the kernel lock debugging options and it came up clean.

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

diff --git a/security/lsm_audit.c b/security/lsm_audit.c
index 69fdf3b..4773b91 100644
--- a/security/lsm_audit.c
+++ b/security/lsm_audit.c
@@ -212,6 +212,7 @@ static void dump_common_audit_data(struct audit_buffer *ab,
 				   struct common_audit_data *a)
 {
 	struct task_struct *tsk = current;
+	char comm[sizeof(tsk->comm)];
 
 	/*
 	 * To keep stack sizes in check force programers to notice if they
@@ -221,7 +222,7 @@ static void dump_common_audit_data(struct audit_buffer *ab,
 	BUILD_BUG_ON(sizeof(a->u) > sizeof(void *)*2);
 
 	audit_log_format(ab, " pid=%d comm=", task_pid_nr(tsk));
-	audit_log_untrustedstring(ab, tsk->comm);
+	audit_log_untrustedstring(ab, get_task_comm(comm, tsk));
 
 	switch (a->type) {
 	case LSM_AUDIT_DATA_NONE:
@@ -282,7 +283,7 @@ static void dump_common_audit_data(struct audit_buffer *ab,
 			pid_t pid = task_pid_nr(tsk);
 			if (pid) {
 				audit_log_format(ab, " pid=%d comm=", pid);
-				audit_log_untrustedstring(ab, tsk->comm);
+				audit_log_untrustedstring(ab, get_task_comm(comm, tsk));
 			}
 		}
 		break;
-- 
1.7.1

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