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

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Message-ID: <239a6775-c514-e752-2520-16668b8bc344@i-love.sakura.ne.jp>
Date:   Wed, 23 Dec 2020 19:11:38 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Christoph Hellwig <hch@....de>
Cc:     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 2020/12/23 16:53, Christoph Hellwig wrote:
> 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.

My question is, in Linux 5.10, sg_check_file_access() for x86 became

static int sg_check_file_access(struct file *filp, const char *caller)
{
	if (filp->f_cred != current_real_cred()) {
		pr_err_once("%s: process %d (%s) changed security contexts after opening file descriptor, this is not allowed.\n",
			caller, task_tgid_vnr(current), current->comm);
		return -EPERM;
	}
	if (0) {
		pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n",
			caller, task_tgid_vnr(current), current->comm);
		return -EACCES;
	}
	return 0;
}

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)" ?

> 
>> 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.
Since "uaccess_kernel()" became "0" for x86, TOMOYO is no longer able to
grant kernel threads implicit permissions to perform socket operations.
Since Eric says "For PF_IO_WORKER kernel threads which are running code on behalf
of a user we want to perform the ordinary permission checks.", I think that
TOMOYO wants to use "(current->flags & (PF_KTHREAD | PF_IO_WORKER)) == PF_KTHREAD"
instead.