linux-kernel - Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for inode_init

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Date:   Fri, 18 Nov 2022 09:31:17 -0800
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Roberto Sassu <roberto.sassu@...weicloud.com>,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        keescook@...omium.org, nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        casey@...aufler-ca.com
Subject: Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

On 11/18/2022 7:10 AM, Mimi Zohar wrote:
> On Fri, 2022-11-18 at 10:14 +0100, Roberto Sassu wrote:
>>>> +static int security_check_compact_xattrs(struct xattr *xattrs,
>>>> +                                     int num_xattrs, int *checked_xattrs)
>>> Perhaps the variable naming is off, making it difficult to read.   So
>>> although this is a static function, which normally doesn't require a
>>> comment, it's definitely needs one.
>> Ok, will improve it.
>>
>>>> +{
>>>> +    int i;
>>>> +
>>>> +    for (i = *checked_xattrs; i < num_xattrs; i++) {
>>> If the number of "checked" xattrs was kept up to date, removing the
>>> empty xattr gaps wouldn't require a loop.  Is the purpose of this loop
>>> to support multiple per LSM xattrs?
>> An LSM might reserve one or more xattrs, but not set it/them (for 
>> example because it is not initialized). In this case, removing the gaps 
>> is needed for all subsequent LSMs.
> Including this sort of info in the function description or as a comment
> in the code would definitely simplify review.
>
> security_check_compact_xattrs() is called in the loop after getting
> each LSM's xattr(s).  Only the current LSMs xattrs need to be
> compressed, yet the loop goes to the maximum number of xattrs each
> time. Just wondering if there is a way of improving it.

At security module registration each module could identify how
many xattrs it uses. That number could be used to limit the range
of the loop. I have to do similar things for the forthcoming LSM
syscalls and module stacking beyond that.


>