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Message-ID: <77ace07b-3f84-46ac-8a29-e782d7f60078@redhat.com>
Date: Wed, 31 Jan 2024 11:54:25 +0100
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Subject: Re: [Syzkaller & bisect] There is BUG: unable to handle kernel paging
 request in fuse_copy_one in intel-6.8-rc2
Content-Language: en-US
To: Mike Rapoport <rppt@kernel.org>
Cc: Pengfei Xu <pengfei.xu@intel.com>, rdunlap@infradead.org,
 akpm@linux-foundation.org, heng.su@intel.com, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, lkp@intel.com
References: <ZbmqEy+8iOREMPmU@xpf.sh.intel.com>
 <ff3058bc-e683-4aa6-833e-6dd6aa92d2bf@redhat.com>
 <ZbolZRw9yFk9jLB6@kernel.org>
From: David Hildenbrand <david@redhat.com>
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On 31.01.24 11:48, Mike Rapoport wrote:
> On Wed, Jan 31, 2024 at 10:15:11AM +0100, David Hildenbrand wrote:
>> On 31.01.24 03:01, Pengfei Xu wrote:
>>> Hi Mike,
>>>
>>> Greeting!
>>>
>>> There is "unable to handle kernel paging request in fuse_copy_one" BUG in
>>> intel-6.8-rc2:
>>>
>>> All detailed info: https://github.com/xupengfe/syzkaller_logs/tree/main/240130_184751_fuse_copy_one
>>> Syzkaller reproduced code: https://github.com/xupengfe/syzkaller_logs/blob/main/240130_184751_fuse_copy_one/repro.c
>>> Syzkaller repro syscall steps: https://github.com/xupengfe/syzkaller_logs/blob/main/240130_184751_fuse_copy_one/repro.prog
>>> Bisect info: https://github.com/xupengfe/syzkaller_logs/blob/main/240130_184751_fuse_copy_one/bisect_info.log
>>> Issue dmesg: https://github.com/xupengfe/syzkaller_logs/blob/main/240130_184751_fuse_copy_one/41bccc98fb7931d63d03f326a746ac4d429c1dd3_dmesg.log
>>> bzImage: https://github.com/xupengfe/syzkaller_logs/raw/main/240130_184751_fuse_copy_one/bzImage_v6.8-rc2.tar.gz
>>> Kconfig(make olddefconfig): https://github.com/xupengfe/syzkaller_logs/blob/main/240130_184751_fuse_copy_one/kconfig_origin
>>>
>>> Bisected and found first bad commit is:
>>> b758fe6df50da mm/secretmem: make it on by default
>>
>> Sounds like we manage to access a page that has its direct map removed --
>> either because it's actual secretmem memory, or because we fail to restore
>> the direct map.
> 
> Judging by the repro code and the registers, it's an access to secretmem.
> 
> The call
> syscall(__NR_write, /*fd=*/r[0], /*arg=*/0x20000040ul, /*len=*/0x50ul);
> 
> will eventually get to
> 
> memcpy(0xffff88800e2afa80, 0xffff8880119a5040, 0x0000000000000010)
> 
> and I think that 0x20000040 in repro address space maps to
> 0xffff8880119a5040 in the direct map.
> 
> So it looks like secretmem actually works :)

Good. How can we prevent triggering BUGs? The write system call should 
fail gracefully (and I thought it would).

-- 
Cheers,

David / dhildenb