Re: [PATCH V1] audit: log xattr args not covered by syscall record

[Casey Schaufler]

On 5/10/2021 4:52 PM, Paul Moore wrote:
> On Mon, May 10, 2021 at 12:30 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>> On 5/7/2021 6:54 PM, Richard Guy Briggs wrote:
>>> On 2021-05-07 14:03, Casey Schaufler wrote:
>>>> On 5/7/2021 12:55 PM, Richard Guy Briggs wrote:
>>>>> The *setxattr syscalls take 5 arguments.  The SYSCALL record only lists
>>>>> four arguments and only lists pointers of string values.  The xattr name
>>>>> string, value string and flags (5th arg) are needed by audit given the
>>>>> syscall's main purpose.
>>>>>
>>>>> Add the auxiliary record AUDIT_XATTR (1336) to record the details not
>>>>> available in the SYSCALL record including the name string, value string
>>>>> and flags.
>>>>>
>>>>> Notes about field names:
>>>>> - name is too generic, use xattr precedent from ima
>>>>> - val is already generic value field name
>>>>> - flags used by mmap, xflags new name
>>>>>
>>>>> Sample event with new record:
>>>>> type=PROCTITLE msg=audit(05/07/2021 12:58:42.176:189) : proctitle=filecap /tmp/ls dac_override
>>>>> type=PATH msg=audit(05/07/2021 12:58:42.176:189) : item=0 name=(null) inode=25 dev=00:1e mode=file,755 ouid=root ogid=root rdev=00:00 obj=unconfined_u:object_r:user_tmp_t:s0 nametype=NORMAL cap_fp=none cap_fi=none cap_fe=0 cap_fver=0 cap_frootid=0
>>>>> type=CWD msg=audit(05/07/2021 12:58:42.176:189) : cwd=/root
>>>>> type=XATTR msg=audit(05/07/2021 12:58:42.176:189) : xattr="security.capability" val=01 xflags=0x0
>>>> Would it be sensible to break out the namespace from the attribute?
>>>>
>>>>      attrspace="security" attrname="capability"
>>> Do xattrs always follow this nomenclature?  Or only the ones we care
>>> about?
>> Xattrs always have a namespace (man 7 xattr) of "user", "trusted",
>> "system" or "security". It's possible that additional namespaces will
>> be created in the future, although it seems unlikely given that only
>> "security" is widely used today.
> Why should audit care about separating the name into two distinct
> fields, e.g. "attrspace" and "attrname", instead of just a single
> "xattr" field with a value that follows the "namespace.attribute"
> format that is commonly seen by userspace?

I asked if it would be sensible. I don't much care myself.

>>>> Why isn't val= quoted?
>>> Good question.  I guessed it should have been since it used
>>> audit_log_untrustedstring(), but even the raw output is unquoted unless
>>> it was converted by auditd to unquoted before being stored to disk due
>>> to nothing offensive found in it since audit_log_n_string() does add
>>> quotes.  (hmmm, bit of a run-on sentence there...)
>>>
>>>> The attribute value can be a .jpg or worse. I could even see it being an eBPF
>>>> program (although That Would Be Wrong) so including it in an audit record could
>>>> be a bit of a problem.
>>> In these cases it would almost certainly get caught by the control
>>> character test audit_string_contains_control() in
>>> audit_log_n_untrustedstring() called from audit_log_untrustedstring()
>>> and deliver it as hex.
>> In that case I'm more concerned with the potential size than with
>> quoting. One of original use cases proposed for xattrs (back in the
>> SGI Irix days) was to attach a bitmap to be used as the icon in file
>> browsers as an xattr. Another was to attach the build instructions
>> and source used to create a binary. None of that is information you'd
>> want to see in a audit record. On the other hand, if the xattr was an
>> eBPF program used to make access control decisions, you would want at
>> least a reference to it in the audit record.
> It would be interesting to see how this code would handle arbitrarily
> large xattr values, or at the very least large enough values to blow
> up the audit record size.
>
> As pointed out elsewhere in this thread, and brought up again above
> (albeit indirectly), I'm guessing we don't really care about *all*
> xattrs, just the "special" xattrs that are security relevant, in which
> case I think we need to reconsider how we collect this data.

Right. And you can't know in advance which xattrs are relevant in the
face of "security=". We might want something like

	bool security_auditable_attribute(struct xattr *xattr)

which returns true if the passed xattr is one that an LSM in the stack
considers relevant. Much like security_ismaclabel(). I don't think that
we can just reuse security_ismaclabel() because there are xattrs that
are security relevant but are not MAC labels. SMACK64TRANSMUTE is one.
File capability sets are another. I also suggest passing the struct xattr
rather than the name because it seems reasonable that an LSM might
consider "user.execbyroot=never" relevant while "user.execbyroot=possible"
isn't.

> I think it would also be very good to see what requirements may be
> driving this work.  Is it purely a "gee, this seems important" or is
> it a hard requirement in a security certification for example.
>