linux-kernel - Re: Does uaccess_kernel() work for detecting kernel thread?

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Message-ID: <20201223075307.GA4185@lst.de>
Date:   Wed, 23 Dec 2020 08:53:07 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Does uaccess_kernel() work for detecting kernel thread?

On Tue, Dec 22, 2020 at 11:39:08PM +0900, Tetsuo Handa wrote:
> For example, if uaccess_kernel() is "false" due to CONFIG_SET_FS=n,
> isn't sg_check_file_access() failing to detect kernel context?

sg_check_file_access does exactly the right thing - fail for all kernel
threads as those can't support the magic it does.

> For another example, if uaccess_kernel() is "false" due to CONFIG_SET_FS=n,
> isn't TOMOYO unexpectedly checking permissions for socket operations?

Can someone explain WTF TOMOYO is even doing there?  A security module
has absolutely no business checking what context it is called from, but
must check the process credentials instead.