linux-kernel - Re: [PATCH 1/6] syscalls: use uaccess_kernel in addr_limit_user

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Date:   Mon, 20 Jul 2020 22:30:30 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Christoph Hellwig <hch@....de>
Cc:     Nick Hu <nickhu@...estech.com>, Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] syscalls: use uaccess_kernel in addr_limit_user_check

On 7/20/20 10:20 PM, Christoph Hellwig wrote:
> On Mon, Jul 20, 2020 at 10:15:37PM -0700, Guenter Roeck wrote:
>>>> -       if (CHECK_DATA_CORRUPTION(uaccess_kernel(),
>>>> +       if (CHECK_DATA_CORRUPTION(!uaccess_kernel(),
>>>>
>>>> How does this work anywhere ?
>>>
>>> No, that is the wrong check - we want to make sure the address
>>> space override doesn't leak to userspace.  The problem is that
>>> armnommu (and m68knommu, but that doesn't call the offending
>>> function) pretends to not have a kernel address space, which doesn't
>>> really work.  Here is the fix I sent out yesterday, which I should
>>> have Cc'ed you on, sorry:
>>>
>>
>> The patch below makes sense, and it does work, but I still suspect
>> that something with your original patch is wrong, or at least suspicious.
>> Reason: My change above (Adding the "!") works for _all_ of my arm boot
>> tests. Or, in other words, it doesn't make a difference if true
>> or false is passed as first parameter of CHECK_DATA_CORRUPTION(), except
>> for nommu systems. Also, unless I am really missing something, your
>> original patch _does_ reverse the logic.
> 
> Well.  segment_eq is in current mainline used in two places:
> 
>  1) to implement uaccess_kernel
>  2) in addr_limit_user_check to implement uaccess_kernel-like
>     semantics using a strange reverse notation
> 
> I think the explanation for your observation is how addr_limit_user_check
> is called on arm.  The addr_limit_check_failed wrapper for it is called
> from assembly code, but only after already checking the addr_limit,
> basically duplicating the segment_eq check.  So for mmu builds it won't
> get called unless we leak the kernel address space override, which
> is a pretty fatal error and won't show up in your boot tests.  The
> only good way to test it is by explicit injecting it using the
> lkdtm module.
> 

Guess I lost it somewhere. Are you saying the check was wrong all along
and your patch fixed it ?

Thanks,
Guenter