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[149.14.88.26]) by smtp.gmail.com with ESMTPSA id q18-20020adf9dd2000000b0033e90e98886sm9006066wre.71.2024.03.13.05.29.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 13 Mar 2024 05:29:13 -0700 (PDT) Message-ID: <420b6d5f-adef-4415-b8cb-16c234dcec38@redhat.com> Date: Wed, 13 Mar 2024 13:29:12 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 06/24] fsverity: pass tree_blocksize to end_enable_verity() Content-Language: en-US To: "Darrick J. Wong" Cc: Matthew Wilcox , Andrey Albershteyn , fsverity@lists.linux.dev, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, chandan.babu@oracle.com, akpm@linux-foundation.org, linux-mm@kvack.org, Eric Biggers References: <20240304191046.157464-2-aalbersh@redhat.com> <20240304191046.157464-8-aalbersh@redhat.com> <20240305005242.GE17145@sol.localdomain> <20240306163000.GP1927156@frogsfrogsfrogs> <20240307220224.GA1799@sol.localdomain> <20240308034650.GK1927156@frogsfrogsfrogs> <20240308044017.GC8111@sol.localdomain> <20240311223815.GW1927156@frogsfrogsfrogs> <9927568e-9f36-4417-9d26-c8a05c220399@redhat.com> <08905bcc-677d-4981-926d-7f407b2f6a4a@redhat.com> <20240312164444.GG1927156@frogsfrogsfrogs> From: David Hildenbrand 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: <20240312164444.GG1927156@frogsfrogsfrogs> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12.03.24 17:44, Darrick J. Wong wrote: > On Tue, Mar 12, 2024 at 04:33:14PM +0100, David Hildenbrand wrote: >> On 12.03.24 16:13, David Hildenbrand wrote: >>> On 11.03.24 23:38, Darrick J. Wong wrote: >>>> [add willy and linux-mm] >>>> >>>> On Thu, Mar 07, 2024 at 08:40:17PM -0800, Eric Biggers wrote: >>>>> On Thu, Mar 07, 2024 at 07:46:50PM -0800, Darrick J. Wong wrote: >>>>>>> BTW, is xfs_repair planned to do anything about any such extra blocks? >>>>>> >>>>>> Sorry to answer your question with a question, but how much checking is >>>>>> $filesystem expected to do for merkle trees? >>>>>> >>>>>> In theory xfs_repair could learn how to interpret the verity descriptor, >>>>>> walk the merkle tree blocks, and even read the file data to confirm >>>>>> intactness. If the descriptor specifies the highest block address then >>>>>> we could certainly trim off excess blocks. But I don't know how much of >>>>>> libfsverity actually lets you do that; I haven't looked into that >>>>>> deeply. :/ >>>>>> >>>>>> For xfs_scrub I guess the job is theoretically simpler, since we only >>>>>> need to stream reads of the verity files through the page cache and let >>>>>> verity tell us if the file data are consistent. >>>>>> >>>>>> For both tools, if something finds errors in the merkle tree structure >>>>>> itself, do we turn off verity? Or do we do something nasty like >>>>>> truncate the file? >>>>> >>>>> As far as I know (I haven't been following btrfs-progs, but I'm familiar with >>>>> e2fsprogs and f2fs-tools), there isn't yet any precedent for fsck actually >>>>> validating the data of verity inodes against their Merkle trees. >>>>> >>>>> e2fsck does delete the verity metadata of inodes that don't have the verity flag >>>>> enabled. That handles cleaning up after a crash during FS_IOC_ENABLE_VERITY. >>>>> >>>>> I suppose that ideally, if an inode's verity metadata is invalid, then fsck >>>>> should delete that inode's verity metadata and remove the verity flag from the >>>>> inode. Checking for a missing or obviously corrupt fsverity_descriptor would be >>>>> fairly straightforward, but it probably wouldn't catch much compared to actually >>>>> validating the data against the Merkle tree. And actually validating the data >>>>> against the Merkle tree would be complex and expensive. Note, none of this >>>>> would work on files that are encrypted. >>>>> >>>>> Re: libfsverity, I think it would be possible to validate a Merkle tree using >>>>> libfsverity_compute_digest() and the callbacks that it supports. But that's not >>>>> quite what it was designed for. >>>>> >>>>>> Is there an ioctl or something that allows userspace to validate an >>>>>> entire file's contents? Sort of like what BLKVERIFY would have done for >>>>>> block devices, except that we might believe its answers? >>>>> >>>>> Just reading the whole file and seeing whether you get an error would do it. >>>>> >>>>> Though if you want to make sure it's really re-reading the on-disk data, it's >>>>> necessary to drop the file's pagecache first. >>>> >>>> I tried a straight pagecache read and it worked like a charm! >>>> >>>> But then I thought to myself, do I really want to waste memory bandwidth >>>> copying a bunch of data? No. I don't even want to incur system call >>>> overhead from reading a single byte every $pagesize bytes. >>>> >>>> So I created 2M mmap areas and read a byte every $pagesize bytes. That >>>> worked too, insofar as SIGBUSes are annoying to handle. But it's >>>> annoying to take signals like that. >>>> >>>> Then I started looking at madvise. MADV_POPULATE_READ looked exactly >>>> like what I wanted -- it prefaults in the pages, and "If populating >>>> fails, a SIGBUS signal is not generated; instead, an error is returned." >>>> >>> >>> Yes, these were the expected semantics :) >>> >>>> But then I tried rigging up a test to see if I could catch an EIO, and >>>> instead I had to SIGKILL the process! It looks filemap_fault returns >>>> VM_FAULT_RETRY to __xfs_filemap_fault, which propagates up through >>>> __do_fault -> do_read_fault -> do_fault -> handle_pte_fault -> >>>> handle_mm_fault -> faultin_page -> __get_user_pages. At faultin_pages, >>>> the VM_FAULT_RETRY is translated to -EBUSY. >>>> >>>> __get_user_pages squashes -EBUSY to 0, so faultin_vma_page_range returns >>>> that to madvise_populate. Unfortunately, madvise_populate increments >>>> its loop counter by the return value (still 0) so it runs in an >>>> infinite loop. The only way out is SIGKILL. >>> >>> That's certainly unexpected. One user I know is QEMU, which primarily >>> uses MADV_POPULATE_WRITE to prefault page tables. Prefaulting in QEMU is >>> primarily used with shmem/hugetlb, where I haven't heard of any such >>> endless loops. >>> >>>> >>>> So I don't know what the correct behavior is here, other than the >>>> infinite loop seems pretty suspect. Is it the correct behavior that >>>> madvise_populate returns EIO if __get_user_pages ever returns zero? >>>> That doesn't quite sound right if it's the case that a zero return could >>>> also happen if memory is tight. >>> >>> madvise_populate() ends up calling faultin_vma_page_range() in a loop. >>> That one calls __get_user_pages(). >>> >>> __get_user_pages() documents: "0 return value is possible when the fault >>> would need to be retried." >>> >>> So that's what the caller does. IIRC, there are cases where we really >>> have to retry (at least once) and will make progress, so treating "0" as >>> an error would be wrong. >>> >>> Staring at other __get_user_pages() users, __get_user_pages_locked() >>> documents: "Please note that this function, unlike __get_user_pages(), >>> will not return 0 for nr_pages > 0, unless FOLL_NOWAIT is used.". >>> >>> But there is some elaborate retry logic in there, whereby the retry will >>> set FOLL_TRIED->FAULT_FLAG_TRIED, and I think we'd fail on the second >>> retry attempt (there are cases where we retry more often, but that's >>> related to something else I believe). >>> >>> So maybe we need a similar retry logic in faultin_vma_page_range()? Or >>> make it use __get_user_pages_locked(), but I recall when I introduced >>> MADV_POPULATE_READ, there was a catch to it. >> >> I'm trying to figure out who will be setting the VM_FAULT_SIGBUS in the >> mmap()+access case you describe above. >> >> Staring at arch/x86/mm/fault.c:do_user_addr_fault(), I don't immediately see >> how we would transition from a VM_FAULT_RETRY loop to VM_FAULT_SIGBUS. >> Because VM_FAULT_SIGBUS would be required for that function to call >> do_sigbus(). > > The code I was looking at yesterday in filemap_fault was: > > page_not_uptodate: > /* > * Umm, take care of errors if the page isn't up-to-date. > * Try to re-read it _once_. We do this synchronously, > * because there really aren't any performance issues here > * and we need to check for errors. > */ > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > error = filemap_read_folio(file, mapping->a_ops->read_folio, folio); > if (fpin) > goto out_retry; > folio_put(folio); > > if (!error || error == AOP_TRUNCATED_PAGE) > goto retry_find; > filemap_invalidate_unlock_shared(mapping); > > return VM_FAULT_SIGBUS; > > Wherein I /think/ fpin is non-null in this case, so if > filemap_read_folio returns an error, we'll do this instead: > > out_retry: > /* > * We dropped the mmap_lock, we need to return to the fault handler to > * re-find the vma and come back and find our hopefully still populated > * page. > */ > if (!IS_ERR(folio)) > folio_put(folio); > if (mapping_locked) > filemap_invalidate_unlock_shared(mapping); > if (fpin) > fput(fpin); > return ret | VM_FAULT_RETRY; > > and since ret was 0 before the goto, the only return code is > VM_FAULT_RETRY. I had speculated that perhaps we could instead do: > > if (fpin) { > if (error) > ret |= VM_FAULT_SIGBUS; > goto out_retry; > } > > But I think the hard part here is that there doesn't seem to be any > distinction between transient read errors (e.g. disk cable fell out) vs. > semi-permanent errors (e.g. verity says the hash doesn't match). > AFAICT, either the read(ahead) sets uptodate and callers read the page, > or it doesn't set it and callers treat that as an error-retry > opportunity. > > For the transient error case VM_FAULT_RETRY makes perfect sense; for the > second case I imagine we'd want something closer to _SIGBUS. Agreed, it's really hard to judge when it's the right time to give up retrying. At least with MADV_POPULATE_READ we should try achieving the same behavior as with mmap()+read access. So if the latter manages to trigger SIGBUS, MADV_POPULATE_READ should return an error. Is there an easy way to for me to reproduce this scenario? -- Cheers, David / dhildenb