linux-kernel - Re: [syzbot] [io-uring?] BUG: unable to handle kernel NULL pointer dereference in __io_remove

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Date:   Mon, 2 Oct 2023 18:12:47 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     syzbot <syzbot+2113e61b8848fa7951d8@...kaller.appspotmail.com>,
        asml.silence@...il.com, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [io-uring?] BUG: unable to handle kernel NULL pointer
 dereference in __io_remove_buffers (2)

On 10/2/23 10:43 AM, Jens Axboe wrote:
> On 10/2/23 8:38 AM, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    ec8c298121e3 Merge tag 'x86-urgent-2023-10-01' of git://gi..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=16ef0ed6680000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=3be743fa9361d5b0
>> dashboard link: https://syzkaller.appspot.com/bug?extid=2113e61b8848fa7951d8
>> compiler:       arm-linux-gnueabi-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>> userspace arch: arm
> 
> I tried the syz repro in the console output, but can't trigger it. It
> also makes very little sense to me... For when there is a reproducer,
> the below would perhaps shed some light on it. We have bl->is_mapped ==
> 1, yet bl->buf_ring is NULL. Probably some artifact of 32-bit arm?

I think this is 32-bit and highmem... The page being mapped into the
kernel is a highmem page, and this won't really fly with having a
permanent ->buf_ring address which we get from page_address().

-- 
Jens Axboe