Subject: Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

From: Pavel Begunkov <[email-masked]>
Date: Mon, 21 Sep 2020 19:31:49 +0300

On 21/09/2020 00:13, David Laight wrote:
> From: Arnd Bergmann
>> Sent: 20 September 2020 21:49
>>
>> On Sun, Sep 20, 2020 at 9:28 PM Andy Lutomirski <[email-masked]> wrote:
>>> On Sun, Sep 20, 2020 at 12:23 PM Matthew Wilcox <[email-masked]> wrote:
>>>>
>>>> On Sun, Sep 20, 2020 at 08:10:31PM +0100, Al Viro wrote:
>>>>> IMO it's much saner to mark those and refuse to touch them from io_uring...
>>>>
>>>> Simpler solution is to remove io_uring from the 32-bit syscall list.
>>>> If you're a 32-bit process, you don't get to use io_uring.  Would
>>>> any real users actually care about that?
>>>
>>> We could go one step farther and declare that we're done adding *any*
>>> new compat syscalls :)
>>
>> Would you also stop adding system calls to native 32-bit systems then?
>>
>> On memory constrained systems (less than 2GB a.t.m.), there is still a
>> strong demand for running 32-bit user space, but all of the recent Arm
>> cores (after Cortex-A55) dropped the ability to run 32-bit kernels, so
>> that compat mode may eventually become the primary way to run
>> Linux on cheap embedded systems.
>>
>> I don't think there is any chance we can realistically take away io_uring
>> from the 32-bit ABI any more now.
> 
> Can't it just run requests from 32bit apps in a kernel thread that has
> the 'in_compat_syscall' flag set?
> Not that i recall seeing the code where it saves the 'compat' nature
> of any requests.
> 
> It is already completely f*cked if you try to pass the command ring
> to a child process - it uses the wrong 'mm'.

And how so? io_uring uses mm of a submitter. The exception is SQPOLL
mode, but it requires CAP_SYS_ADMIN or CAP_SYS_NICE anyway.

> I suspect there are some really horrid security holes in that area.
> 
> 	David.
> 
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-- 
Pavel Begunkov