Re: [PATCH] audit: check the length of userspace generated audit records

On Mon, Apr 20, 2020 at 5:56 PM Richard Guy Briggs&lt;rgb(a)redhat.com&gt; 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&lt;paul(a)paul-moore.com&gt; >> --- >> 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?