Message-ID: <a6dbc5b3-b12e-36b4-0aef-f319264d6e8f@redhat.com>
Date:   Wed, 1 Sep 2021 10:28:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
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Subject: Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

On 27.08.21 00:13, Eric W. Biederman wrote:
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 26.08.21 19:48, Andy Lutomirski wrote:
>>> On Fri, Aug 13, 2021, at 5:54 PM, Linus Torvalds wrote:
>>>> On Fri, Aug 13, 2021 at 2:49 PM Andy Lutomirski <luto@...nel.org> wrote:
>>>>>
>>>>> I’ll bite.  How about we attack this in the opposite direction: remove the deny write mechanism entirely.
>>>>
>>>> I think that would be ok, except I can see somebody relying on it.
>>>>
>>>> It's broken, it's stupid, but we've done that ETXTBUSY for a _loong_ time.
>>>
>>> Someone off-list just pointed something out to me, and I think we should push harder to remove ETXTBSY.  Specifically, we've all been focused on open() failing with ETXTBSY, and it's easy to make fun of anyone opening a running program for write when they should be unlinking and replacing it.
>>>
>>> Alas, Linux's implementation of deny_write_access() is correct^Wabsurd, and deny_write_access() *also* returns ETXTBSY if the file is open for write.  So, in a multithreaded program, one thread does:
>>>
>>> fd = open("some exefile", O_RDWR | O_CREAT | O_CLOEXEC);
>>> write(fd, some stuff);
>>>
>>> <--- problem is here
>>>
>>> close(fd);
>>> execve("some exefile");
>>>
>>> Another thread does:
>>>
>>> fork();
>>> execve("something else");
>>>
>>> In between fork and execve, there's another copy of the open file description, and i_writecount is held, and the execve() fails.  Whoops.  See, for example:
>>>
>>> https://github.com/golang/go/issues/22315
>>>
>>> I propose we get rid of deny_write_access() completely to solve this.
>>>
>>> Getting rid of i_writecount itself seems a bit harder, since a handful of filesystems use it for clever reasons.
>>>
>>> (OFD locks seem like they might have the same problem.  Maybe we should have a clone() flag to unshare the file table and close close-on-exec things?)
>>>
>>
>> It's not like this issue is new (^2017) or relevant in practice. So no
>> need to hurry IMHO. One step at a time: it might make perfect sense to
>> remove ETXTBSY, but we have to be careful to not break other user
>> space that actually cares about the current behavior in practice.
> 
> It is an old enough issue that I agree there is no need to hurry.
> 
> I also ran into this issue not too long ago when I refactored the
> usermode_driver code.  My challenge was not being in userspace
> the delayed fput was not happening in my kernel thread.  Which meant
> that writing the file, then closing the file, then execing the file
> consistently reported -ETXTBSY.
> 
> The kernel code wound up doing:
> 	/* Flush delayed fput so exec can open the file read-only */
> 	flush_delayed_fput();
> 	task_work_run();
> 
> As I read the code the delay for userspace file descriptors is
> always done with task_work_add, so userspace should not hit
> that kind of silliness, and should be able to actually close
> the file descriptor before the exec.
> 
> 
> On the flip side, I don't know how anything can depend upon getting an
> -ETXTBSY.  So I don't think there is any real risk of breaking userspace
> if we remove it.

At least in LTP, we have two test cases testing exactly that behavior:

testcases/kernel/syscalls/creat/creat07.c
testcases/kernel/syscalls/execve/execve04.c


-- 
Thanks,

David / dhildenb

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