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Subject: Re: Slab out of bounds in setxattr
To:     shankarapailoor <shankarapailoor@gmail.com>
Cc:     jfs-discussion@lists.sourceforge.net, linux-kernel@vger.kernel.org
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From:   Dave Kleikamp <dave.kleikamp@oracle.com>
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Date:   Mon, 4 Jun 2018 13:39:37 -0500
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On 06/04/2018 01:30 PM, shankarapailoor wrote:
> Hi Dave,
> 
> Attached is my proposed patch. It solves the problem as you suggest
> and I don't see the KASAN complaint.

That looks good to me. Add a description and a Signed-off-by: and I'll
get it pushed upstream.

Thanks for finding this.

Shaggy

> 
> Regards,
> Shankara
> 
> On Mon, Jun 4, 2018 at 11:24 AM, Dave Kleikamp <dave.kleikamp@oracle.com> wrote:
>> On 06/01/2018 11:06 PM, shankarapailoor wrote:
>>> Hi,
>>>
>>> Looking at the crash some more, it seems that if value_len > PAGE_SIZE
>>> then e_buf->max_size is rounded up nearest page size [1]. If a new
>>> attribute is added with value_len < e_buf->max_size - EA_SIZE(ea) then
>>> no new space is allocated for the attiribute list [2] and this
>>> triggers the KASAN slab out of bounds error. This is the case in the C
>>> repro I provided.
>>
>> I see the problem. It looks like we should be calculating max_size
>> earlier and using that to call kmalloc(). (xattr.c#496)
>>
>> Shaggy
>>>
>>>
>>> 1. https://elixir.bootlin.com/linux/v4.17-rc7/source/fs/jfs/xattr.c#L501
>>> 2. https://elixir.bootlin.com/linux/v4.17-rc7/source/fs/jfs/xattr.c#L723
>>>
>>> On Fri, Jun 1, 2018 at 1:52 PM, shankarapailoor
>>> <shankarapailoor@gmail.com> wrote:
>>>> Hi Dave et al,
>>>>
>>>> I have been fuzzing linux 4.17-rc4 with JFS using Syzkaller  KASAN:
>>>> slab-out-of-bounds in jfs_xattr.
>>>>
>>>> Attached are my kernel configs and a C reproducer. In the first
>>>> setxattr call it appears that length is much larger than the name. In
>>>> __jfs_setxattr, I don't see where the length is checked against the
>>>> actual value length.
>>>>
>>>> Regards,
>>>> Shankara Pailoor