audit: fix two bugs in the new execve audit code

copy_from_user() returns the number of bytes not copied, hence 0 is the
expected output.

axi->mm might not be valid anymore when not equal to current->mm, do not
dereference before checking that - thanks to Al for spotting that.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Steve Grubb <sgrubb@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index bde1124d5908..a777d3761416 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -824,12 +824,14 @@ static void audit_log_execve_info(struct audit_buffer *ab,
 {
         int i;
         long len, ret;
-        const char __user *p = (const char __user *)axi->mm->arg_start;
+        const char __user *p;
         char *buf;
 
         if (axi->mm != current->mm)
                 return; /* execve failed, no additional info */
 
+        p = (const char __user *)axi->mm->arg_start;
+
         for (i = 0; i < axi->argc; i++, p += len) {
                 len = strnlen_user(p, MAX_ARG_STRLEN);
                 /*
@@ -855,7 +857,7 @@ static void audit_log_execve_info(struct audit_buffer *ab,
                  * copied them here, and the mm hasn't been exposed to user-
                  * space yet.
                  */
-                if (!ret) {
+                if (ret) {
                         WARN_ON(1);
                         send_sig(SIGKILL, current, 0);
                 }