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

[Casey Schaufler]

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 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 seems that you might want to leave it up to the LSMs to determine which xattr
>> values are audited. An SELinux system may want to see "security.selinux" values,
>> but it probably doesn't care about "security.SMACK64TRANSMUTE" values.
> Are you suggesting that any that don't follow this nomenclature are
> irrelevant or harmless at best and are not worth recording?

Nope. I'm saying that unless an xattr is going to be used for security
related purposes it isn't security relevant and hence shouldn't go in
the audit trail. The "security" namespace indication may not be sufficient
to make that determination, although it's a pretty good indicator today.
As I pointed out, setting an attribute used by Smack on an SELinux system
is not security relevant.

There is nothing preventing an LSM from using "user" namespace attributes.
I could imagine a security policy that looks a user supplied attributes
(user.runasroot=never) and restricts when a program can be used. Thus, it
needs to be up to the LSM to decide if an attribute should be audited. We
have to leave ACLs as an exception, of course, because they don't (and in
their current form can't) use the LSM infrastructure.

> This sounds like you are thinking about your LSM stacking patchset that
> issues a new record for each LSM attribute if there is more than one.
> And any system that has multiple LSMs active should be able to indicate
> all of interest.

There's some of that, but realize that Smack already uses multiple xattrs,
none of which are "security.selinux". Logging setting of "security.SMACK64EXEC"
makes sense. Logging "security.MyDogHasNoNose" does not. On the other hand,
if Yama decided to start using that attribute you'd have to look for it as well.