Received-SPF: pass (google.com: domain of linux-kernel+bounces-186691-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <ff978ce9-bd22-4ee2-ba14-6dd8ee0b05dd@redhat.com>
Date: Wed, 22 May 2024 22:25:55 +0200
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: 6.10/bisected/regression - commit 8430557fc584 cause warning at
 mm/page_table_check.c:198 __page_table_check_ptes_set+0x306
To: Peter Xu <peterx@redhat.com>
Cc: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>,
 Pavel Tatashin <pasha.tatashin@soleen.com>, axelrasmussen@google.com,
 nadav.amit@gmail.com, Andrew Morton <akpm@linux-foundation.org>,
 Linux Memory Management List <linux-mm@kvack.org>,
 Linux List Kernel Mailing <linux-kernel@vger.kernel.org>
References: <CABXGCsMB9A8-X+Np_Q+fWLURYL_0t3Y-MdoNabDM-Lzk58-DGA@mail.gmail.com>
 <Zk0HxVODITGKqYCw@x1n>
 <CABXGCsNbcMn0Z0RudFrBW78rZPE+cDY+f9r+yKf_AZwJZUOrQg@mail.gmail.com>
 <Zk0UA6wABOB9X_Dx@x1n>
 <CABXGCsOZnxrSHd0y6QrFhzAiY-uTJiRSmo__C_P8Y2qjFV6bRA@mail.gmail.com>
 <Zk0h0V8kvZRKu6F4@x1n> <a3d54407-87aa-4f59-adac-c9b79fe1ecef@redhat.com>
 <Zk4MsGxhP5x5aURG@x1n> <03faa624-1685-4a21-81fc-cc9e8b760e97@redhat.com>
 <Zk4Y9tU7pOzU0lw1@x1n>
From: David Hildenbrand <david@redhat.com>
Content-Language: en-US
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <Zk4Y9tU7pOzU0lw1@x1n>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit

On 22.05.24 18:10, Peter Xu wrote:
> On Wed, May 22, 2024 at 05:34:21PM +0200, David Hildenbrand wrote:
>> On 22.05.24 17:18, Peter Xu wrote:
>>> On Wed, May 22, 2024 at 09:48:51AM +0200, David Hildenbrand wrote:
>>>> On 22.05.24 00:36, Peter Xu wrote:
>>>>> On Wed, May 22, 2024 at 03:21:04AM +0500, Mikhail Gavrilov wrote:
>>>>>> On Wed, May 22, 2024 at 2:37 AM Peter Xu <peterx@redhat.com> wrote:
>>>>>>> Hmm I still cannot reproduce.  Weird.
>>>>>>>
>>>>>>> Would it be possible for you to identify which line in debug_vm_pgtable.c
>>>>>>> triggered that issue?
>>>>>>>
>>>>>>> I think it should be some set_pte_at() but I'm not sure, as there aren't a
>>>>>>> lot and all of them look benign so far.  It could be that I missed
>>>>>>> something important.
>>>>>>
>>>>>> I hope it's helps:
>>>>>
>>>>> Thanks for offering this, it's just that it doesn't look coherent with what
>>>>> was reported for some reason.
>>>>>
>>>>>>
>>>>>>> sh /usr/src/kernels/(uname -r)/scripts/faddr2line /lib/debug/lib/modules/(uname -r)/vmlinux debug_vm_pgtable+0x1c04
>>>>>> debug_vm_pgtable+0x1c04/0x3360:
>>>>>> native_ptep_get_and_clear at arch/x86/include/asm/pgtable_64.h:94
>>>>>> (inlined by) ptep_get_and_clear at arch/x86/include/asm/pgtable.h:1262
>>>>>> (inlined by) ptep_clear at include/linux/pgtable.h:509
>>>>>
>>>>> This is a pte_clear(), and pte_clear() shouldn't even do the set() checks,
>>>>> and shouldn't stumble over what I added.
>>>>>
>>>>> IOW, it doesn't match with the real stack dump previously:
>>>>>
>>>>> [    5.581003]  ? __page_table_check_ptes_set+0x306/0x3c0
>>>>> [    5.581274]  ? __pfx___page_table_check_ptes_set+0x10/0x10
>>>>> [    5.581544]  ? __pfx_check_pgprot+0x10/0x10
>>>>> [    5.581806]  set_ptes.constprop.0+0x66/0xd0
>>>>> [    5.582072]  ? __pfx_set_ptes.constprop.0+0x10/0x10
>>>>> [    5.582333]  ? __pfx_pte_val+0x10/0x10
>>>>> [    5.582595]  debug_vm_pgtable+0x1c04/0x3360
>>>>>
>>>>
>>>> Staring at pte_clear_tests():
>>>>
>>>> #ifndef CONFIG_RISCV
>>>> 	pte = __pte(pte_val(pte) | RANDOM_ORVALUE);
>>>> #endif
>>>> 	set_pte_at(args->mm, args->vaddr, args->ptep, pte);
>>>>
>>>> So we set random PTE bits, probably setting the present, uffd and write bit
>>>> at the same time. That doesn't make too much sense when we want to perform
>>>> that such combinations cannot exist.
>>>
>>> Here the issue is I don't think it should set W bit anyway, as we init
>>> page_prot to be RWX but !shared:
>>>
>>> 	args->page_prot          = vm_get_page_prot(VM_ACCESS_FLAGS);
>>>
>>> On x86_64 (Mikhail's system) it should have W bit cleared afaict, meanwhile
>>> the RANDOM_ORVALUE won't touch bit W due to S390_SKIP_MASK (which contains
>>> bit W / bit 1, which is another "accident"..).  Then even if with that it
>>> should not trigger..  I think that's also why I cannot reproduce this
>>> problem locally.
>>
>> Why oh why are skip mask applied independently of the architecture.
>>
>> While _PAGE_RW should indeed be masked out by RANDOM_ORVALUE.
>>
>> But with shadow stacks we consider a PTE writable (see
>> pte_write()->pte_shstk()) if
>> (1) X86_FEATURE_SHSTK is enabled
>> (2) _PAGE_RW is clear
>> (3) _PAGE_DIRTY is set
>>
>> _PAGE_DIRTY is bit 6.
>>
>> Likely your CPU does not support shadow stacks.
> 
> Good point.  My host has it, but I tested in the VM which doesn't.  I
> suppose we can wait and double check whether Mikhail should see the issue
> went away with that patch provided.
> 
> In this case, instead of keep fiddling with random bits to apply and
> further work on top of per-arch random bits, I'd hope we can simply drop
> that random mechanism as I don't think it'll be pxx_none() now.  I attached
> a patch I plan to post. Does it look reasonable?

I doubt that randomness ever helped in finding a BUG. Clearing is just 
too simple ... but I might just be wrong :)

I'd vote for removing that, this will likely not be the last issue we 
run into once we add more sanity checks during set_pte_at().

-- 
Cheers,

David / dhildenb