linux-kernel - Re: block: DMA alignment of IO buffer allocated from slab

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 24 Sep 2018 12:46:27 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Ming Lei <ming.lei@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Ming Lei <tom.leiming@...il.com>,
        linux-block <linux-block@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        "open list:XFS FILESYSTEM" <linux-xfs@...r.kernel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>, Christoph Lameter <cl@...ux.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: block: DMA alignment of IO buffer allocated from slab

On 09/24/2018 01:42 AM, Ming Lei wrote:
> On Fri, Sep 21, 2018 at 03:04:18PM +0200, Vitaly Kuznetsov wrote:
>> Christoph Hellwig <hch@....de> writes:
>>
>>> On Wed, Sep 19, 2018 at 05:15:43PM +0800, Ming Lei wrote:
>>>> 1) does kmalloc-N slab guarantee to return N-byte aligned buffer?  If
>>>> yes, is it a stable rule?
>>>
>>> This is the assumption in a lot of the kernel, so I think if somethings
>>> breaks this we are in a lot of pain.

This assumption is not correct. And it's not correct at least from the beginning of the
git era, which is even before SLUB allocator appeared. With CONFIG_DEBUG_SLAB=y
the same as with CONFIG_SLUB_DEBUG_ON=y kmalloc return 'unaligned' objects.
The guaranteed arch-and-config-independent alignment of kmalloc() result is "sizeof(void*)".

If objects has higher alignment requirement, the could be allocated via specifically created kmem_cache.


> 
> Even some of buffer address is _not_ L1 cache size aligned, this way is
> totally broken wrt. DMA to/from this buffer.
> 
> So this issue has to be fixed in slab debug side.
> 

Well, this definitely would increase memory consumption. Many (probably most) of the kmalloc()
users doesn't need such alignment, why should they pay the cost?