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Mon, 20 Apr 2020 16:56:30 -0700 (PDT) Received: from [192.168.0.137] ([216.158.248.173]) by smtp.gmail.com with ESMTPSA id h82sm383800ila.14.2020.04.20.16.56.30 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Apr 2020 16:56:30 -0700 (PDT) Subject: Re: [PATCH] audit: check the length of userspace generated audit records To: linux-audit@redhat.com References: <158741856073.246860.5955712228915579.stgit@chester> <20200420215615.chh4c5ytxumadtt7@madcap2.tricolour.ca> From: Lenny Bruzenak Message-ID: <728a2bfa-a5f2-4178-49f1-405adef62ee4@magitekltd.com> Date: Mon, 20 Apr 2020 17:56:27 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-loop: linux-audit@redhat.com X-BeenThere: linux-audit@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: Linux Audit Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linux-audit-bounces@redhat.com Errors-To: linux-audit-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/mixed; boundary="===============9136615526811011628==" This is a multi-part message in MIME format. --===============9136615526811011628== Content-Language: en-US Content-Type: multipart/alternative; boundary="------------C6E58AF2CE9CAC9253CECB6A" This is a multi-part message in MIME format. --------------C6E58AF2CE9CAC9253CECB6A Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable On 4/20/20 5:29 PM, Paul Moore wrote: > On Mon, Apr 20, 2020 at 5:56 PM Richard Guy Briggs 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 >>> --- >>> 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 !=3D AUDIT_USER_AVC) >>> return 0; >>> + /* exit early if there isn't at least one character to pr= int */ >>> + 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] =3D '\0' and str poin= ts >> to a location with a null that prints nothing between the single quotes >> of "msg=3D''". 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=20 string is sent, and handled in the lib call(s), I believe it will have=20 minimum contextual info, e.g. "exe=3D... hostname=3D... ", etc. I can't think of a valid reason myself, assuming IIUC. LCB --=20 LC Bruzenak MagitekLTD --------------C6E58AF2CE9CAC9253CECB6A Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

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

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

On 2020-04-20 17:36, Paul Mo=
ore wrote:

Commit 756125289285 ("audi=
t: 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 aud=
it_receive_msg()")
Reported-by: syzb=
ot+49e69b4d71a420ceda3e@syzkaller.appspotmail.com
Signed-off-by: Paul Moore <paul@paul-moore.com><=
/a>
---
 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, str=
uct nlmsghdr *nlh)
      case AUDIT_FIRST_USER_MSG2 ... AUDIT_LAST_USER_MSG2:
              if (!audit_enabled && msg_type !=3D 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 r=
ecord even if the message is empty?  If a
len of 1 is passed in, it will properly set str[0] =3D '\0' and str points
to a location with a null that prints nothing between the single quotes
of "msg=3D''".  So, I think that should be "if (data_len < 1)".

Am I missing something?

I've got no problem with allow=
ing 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=3D... hostname=3D... ", etc.

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

LCB


--=20
LC Bruzenak
MagitekLTD
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