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[91.12.97.173]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4361b09fec0sm12034785e9.4.2024.12.10.13.43.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Dec 2024 13:43:57 -0800 (PST) Message-ID: <63ae2ae4-b023-4802-9b34-a2c0d272f6d7@redhat.com> Date: Tue, 10 Dec 2024 22:43:55 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] fork: avoid inappropriate uprobe access to invalid mm To: Lorenzo Stoakes Cc: Peter Zijlstra , Oleg Nesterov , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Kan Liang , Masami Hiramatsu , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Andrew Morton , "Liam R . Howlett" , Vlastimil Babka , Jann Horn , linux-mm@kvack.org, Peng Zhang , syzbot+2d788f4f7cb660dac4b7@syzkaller.appspotmail.com References: <20241210172412.52995-1-lorenzo.stoakes@oracle.com> <3bd43f87-571a-4bc6-b068-69056ec18d85@redhat.com> From: David Hildenbrand 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10.12.24 21:59, Lorenzo Stoakes wrote: > On Tue, Dec 10, 2024 at 08:35:30PM +0100, David Hildenbrand wrote: >> On 10.12.24 18:24, Lorenzo Stoakes wrote: >>> If dup_mmap() encounters an issue, currently uprobe is able to access the >>> relevant mm via the reverse mapping (in build_map_info()), and if we are >>> very unlucky with a race window, observe invalid XA_ZERO_ENTRY state which >>> we establish as part of the fork error path. >>> >>> This occurs because uprobe_write_opcode() invokes anon_vma_prepare() which >>> in turn invokes find_mergeable_anon_vma() that uses a VMA iterator, >>> invoking vma_iter_load() which uses the advanced maple tree API and thus is >>> able to observe XA_ZERO_ENTRY entries added to dup_mmap() in commit >>> d24062914837 ("fork: use __mt_dup() to duplicate maple tree in >>> dup_mmap()"). >>> >>> This change was made on the assumption that only process tear-down code >>> would actually observe (and make use of) these values. However this very >>> unlikely but still possible edge case with uprobes exists and unfortunately >>> does make these observable. >>> >>> The uprobe operation prevents races against the dup_mmap() operation via >>> the dup_mmap_sem semaphore, which is acquired via uprobe_start_dup_mmap() >>> and dropped via uprobe_end_dup_mmap(), and held across >>> register_for_each_vma() prior to invoking build_map_info() which does the >>> reverse mapping lookup. >>> >>> Currently these are acquired and dropped within dup_mmap(), which exposes >>> the race window prior to error handling in the invoking dup_mm() which >>> tears down the mm. >>> >>> We can avoid all this by just moving the invocation of >>> uprobe_start_dup_mmap() and uprobe_end_dup_mmap() up a level to dup_mm() >>> and only release this lock once the dup_mmap() operation succeeds or clean >>> up is done. >> >> What I understand is: we need to perform the uprobe_end_dup_mmap() after the >> mmput(). > > Ack yes. > >> >> I assume/hope that we cannot see another mmget() before we return here. In >> that case, this LGTM. > > We are dealing with a tiny time window and brief rmap availability, so it's hard > to say that's impossible. You also have to have failed to allocate really very > small amounts of memory, so we are talking lottery odds for this to even happen > in the first instance :) Yes, likely the error injection framework is one of the only reliable ways to trigger that :) > > I mean the syzkaller report took a year or so to hit it, and had to do > fault injection to do so. Ah, there it is: "fault injection" :D > > Also it's not impossible that there are other means of accessing the mm > contianing XA_ZERO_ENTRY items through other means (I believe Liam was looking > into this). > > However this patch is intended to at least eliminate the most proximate obvious > case with as simple a code change as possible. > > Ideally we'd somehow mark the mm as being inaccessible somehow, but MMF_ flags > are out, and the obvious one to extend to mean this here, MMF_UNSTABLE, may > interact with oomk logic in some horrid way. > >> >> -- >> Cheers, >> >> David / dhildenb >> > > So overall this patch is a relatively benign attempt to deal with the most > obvious issue with no apparent cost, but doesn't really rule out the need > to do more going forward... Maybe add a bit of that to the patch description. Like "Fixes the reproducer, but likely there is more we'll tackle separately", your call. Thanks for the details! -- Cheers, David / dhildenb