On 4/20/20 5:29 PM, Paul Moore wrote:

On Mon, Apr 20, 2020 at 5:56 PM Richard Guy Briggs <rgb@redhat.com> wrote:

On 2020-04-20 17:36, Paul Moore wrote:

Commit 756125289285 ("audit: always check the netlink payload length
in audit_receive_msg()") fixed a number of missing message length
checks, but forgot to check the length of userspace generated audit
records.  The good news is that you need CAP_AUDIT_WRITE to submit
userspace audit records, which is generally only given to trusted
processes, so the impact should be limited.

Cc: stable@vger.kernel.org
Fixes: 756125289285 ("audit: always check the netlink payload length in audit_receive_msg()")
Reported-by: syzbot+49e69b4d71a420ceda3e@syzkaller.appspotmail.com
Signed-off-by: Paul Moore <paul@paul-moore.com>
---
 kernel/audit.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/audit.c b/kernel/audit.c
index b69c8b460341..87f31bf1f0a0 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -1326,6 +1326,9 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
      case AUDIT_FIRST_USER_MSG2 ... AUDIT_LAST_USER_MSG2:
              if (!audit_enabled && msg_type != AUDIT_USER_AVC)
                      return 0;
+             /* exit early if there isn't at least one character to print */
+             if (data_len < 2)
+                     return -EINVAL;

Don't we want to issue the record even if the message is empty?  If a
len of 1 is passed in, it will properly set str[0] = '\0' and str points
to a location with a null that prints nothing between the single quotes
of "msg=''".  So, I think that should be "if (data_len < 1)".

Am I missing something?

I've got no problem with allowing an empty message so long as there is
a valid use for such a message.  Can anyone think of a valid reason
for having an empty userspace generated record?

Not really. Using the libaudit interface(s), even if an empty message string is sent, and handled in the lib call(s), I believe it will have minimum contextual info, e.g. "exe=... hostname=... ", etc.

I can't think of a valid reason myself, assuming IIUC.

LCB

-- 
LC Bruzenak
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