linux-kernel - Re: [PATCH 0/2] mm: make copy_to_kernel_nofault() not fault on user addresses

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Message-ID: <861d448c-ce1d-4b74-87eb-9b211dfebbb1@redhat.com>
Date: Mon, 2 Sep 2024 10:56:27 +0200
From: David Hildenbrand <david@...hat.com>
To: Omar Sandoval <osandov@...ndov.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Christoph Hellwig <hch@....de>, x86@...nel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-um@...ts.infradead.org, kernel-team@...com
Subject: Re: [PATCH 0/2] mm: make copy_to_kernel_nofault() not fault on user
 addresses

On 02.09.24 08:31, Omar Sandoval wrote:
> On Mon, Sep 02, 2024 at 08:19:33AM +0200, Christophe Leroy wrote:
>>
>>
>> Le 02/09/2024 à 07:31, Omar Sandoval a écrit :
>>> [Vous ne recevez pas souvent de courriers de osandov@...ndov.com. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> From: Omar Sandoval <osandov@...com>
>>>
>>> Hi,
>>>
>>> I hit a case where copy_to_kernel_nofault() will fault (lol): if the
>>> destination address is in userspace and x86 Supervisor Mode Access
>>> Prevention is enabled. Patch 2 has the details and the fix. Patch 1
>>> renames a helper function so that its use in patch 2 makes more sense.
>>> If the rename is too intrusive, I can drop it.
>>
>> The name of the function is "copy_to_kernel". If the destination is a user
>> address, it is not a copy to kernel but a copy to user and you already have
>> the function copy_to_user() for that. copy_to_user() properly handles SMAP.
> 
> I'm not trying to copy to user. I am (well, KDB is) trying to copy to an
> arbitrary address, and I want it to return an error instead of crashing
> if the address is not a valid kernel address. As far as I can tell, that
> is the whole point of copy_to_kernel_nofault().

The thing is that you (well, KDB) triggers something that would be 
considered a real BUG when triggered from "ordinary" (non-debugging) code.

But now I am confused: "if the destination address is in userspace" does 
not really make sense in the context of KDB, no?

   [15]kdb> mm 0 1234
   [   94.652476] BUG: kernel NULL pointer dereference, address: 
0000000000000000

Why is address 0 in "user space"? "Which" user space?

Isn't the problem here that KDB lets you blindly write to any 
non-existing memory address?


Likely it should do some proper filtering like we do in fs/proc/kcore.c:

Take a look at the KCORE_RAM case where we make sure the page exists, is 
online and may be accessed. Only then, we trigger a 
copy_from_kernel_nofault(). Note that the KCORE_USER is a corner case 
only for some special thingies on x86 (vsyscall), and can be ignored for 
our case here.

-- 
Cheers,

David / dhildenb