Message-ID: <c5a11d40-9081-dfad-3d5a-7d46ba764dd5@schaufler-ca.com>
Date:   Tue, 7 Mar 2023 09:13:48 -0800
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Subject: Re: [PATCH v6 04/11] LSM: syscalls for current process attributes
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To:     =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>,
        paul@paul-moore.com, linux-security-module@vger.kernel.org
Cc:     jmorris@namei.org, keescook@chromium.org,
        john.johansen@canonical.com, penguin-kernel@i-love.sakura.ne.jp,
        stephen.smalley.work@gmail.com, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, casey@schaufler-ca.com
References: <20230222200838.8149-1-casey@schaufler-ca.com>
 <20230222200838.8149-5-casey@schaufler-ca.com>
 <83b6310f-6101-2cdf-ec41-b850fe66b1c2@digikod.net>
From:   Casey Schaufler <casey@schaufler-ca.com>
In-Reply-To: <83b6310f-6101-2cdf-ec41-b850fe66b1c2@digikod.net>
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On 3/7/2023 3:56 AM, Mickaël Salaün wrote:
> Let's say an LSM need to pass a file descriptor instead of a text
> value. Would that be possible or would it need to use another interface?

You could use this interface. LSM_ATTR_MAGICFD would have a
ctx_len = sizeof(fd) and the value in ctx. The underlying
plumbing is another matter entirely. It's likely you'd need
to provide more information in the ctx than the fd, but I
couldn't say what it would be, and I won't speculate.

I would not advocate such a use, as I am not now nor have
ever been a fan of passed file descriptors.

>
>
> On 22/02/2023 21:08, Casey Schaufler wrote:
>> Create a system call lsm_get_self_attr() to provide the security
>> module maintained attributes of the current process.
>> Create a system call lsm_set_self_attr() to set a security
>> module maintained attribute of the current process.
>> Historically these attributes have been exposed to user space via
>> entries in procfs under /proc/self/attr.
>>
>> The attribute value is provided in a lsm_ctx structure. The structure
>> identifys the size of the attribute, and the attribute value. The format
>> of the attribute value is defined by the security module. A flags field
>> is included for LSM specific information. It is currently unused and
>> must
>> be 0. The total size of the data, including the lsm_ctx structure and
>> any
>> padding, is maintained as well.
>>
>> struct lsm_ctx {
>>          __u64   id;
>>          __u64   flags;
>>          __u64   len;
>>          __u64   ctx_len;
>>          __u8    ctx[];
>> };
>>
>> Two new LSM hooks are used to interface with the LSMs.
>> security_getselfattr() collects the lsm_ctx values from the
>> LSMs that support the hook, accounting for space requirements.
>> security_setselfattr() identifies which LSM the attribute is
>> intended for and passes it along.