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

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Date:   Tue, 5 Jan 2021 08:59:46 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jens Axboe <axboe@...nel.dk>,
        Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Does uaccess_kernel() work for detecting kernel thread?

On Wed, Dec 23, 2020 at 07:11:38PM +0900, Tetsuo Handa wrote:
> due to commit 5e6e9852d6f76e01 ("uaccess: add infrastructure for kernel
> builds with set_fs()") and follow up changes. Don't we need to change this
> "uaccess_kernel()" with "(current->flags & PF_KTHREAD)" ?

No.  The real problem here is that when a this funtion is called under
set_fs it allows kernel memory access for all user pointers, and due to
the indirection in the playload allows reading or changing kernel
memory.  A kthread does not have that issue.

> >> 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.
> > 
> 
> TOMOYO distinguishes userspace processes and kernel threads, and grants
> kernel threads implicit permissions to perform socket operations.

And this is the problem we need to fix.  A kernel thread can't just have
implicit permissions only because it is a kernel thread.  Think of e.g.
the io_uring service threads.