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

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Date:   Tue, 5 Jan 2021 19:11:03 +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 2021/01/05 16:59, Christoph Hellwig wrote:
> 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

Called by "who" ?
Called by "a userspace process" ?
Called by "a kernel thread" ?
Called by "an io_uring service thread" ?

>                                                                  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.

If this uaccess_kernel() is intended to reject calling this function from
"a userspace process", uaccess_kernel() is failing to reject because
uaccess_kernel() is always "false" for x86.

If this uaccess_kernel() is intended to reject calling this function from
"a kernel thread", uaccess_kernel() is failing to reject because
uaccess_kernel() is always "false" for x86.

If this uaccess_kernel() is intended to reject calling this function from
"an io_uring service thread", uaccess_kernel() is failing to reject because
uaccess_kernel() is always "false" for x86.

What does uaccess_kernel() in sg_check_file_access() (and uhid_char_write(),
ib_safe_file_access(), bpfilter_process_sockopt() etc.) want to check?

> 
>>>> 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.

We can use (current->flags & (PF_KTHREAD | PF_IO_WORKER)) == PF_KTHREAD like
https://lkml.kernel.org/r/dacfb329-de66-d0cf-dcf9-f030ea1370de@schaufler-ca.com does
in order to exclude e.g. the io_uring service threads, can't we?