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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4521370936esm132220275e9.20.2025.06.10.00.30.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Jun 2025 00:30:58 -0700 (PDT) Message-ID: Date: Tue, 10 Jun 2025 09:30:57 +0200 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 3/4] userfaultfd: prevent unregistering VMAs through a different userfaultfd To: Tal Zussman , Andrew Morton , Peter Xu , "Jason A. Donenfeld" , Alexander Viro , Christian Brauner , Jan Kara , Andrea Arcangeli Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20250607-uffd-fixes-v2-0-339dafe9a2fe@columbia.edu> <20250607-uffd-fixes-v2-3-339dafe9a2fe@columbia.edu> 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: <20250607-uffd-fixes-v2-3-339dafe9a2fe@columbia.edu> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 07.06.25 08:40, Tal Zussman wrote: > Currently, a VMA registered with a uffd can be unregistered through a > different uffd associated with the same mm_struct. > > The existing behavior is slightly broken and may incorrectly reject > unregistering some VMAs due to the following check: > > if (!vma_can_userfault(cur, cur->vm_flags, wp_async)) > goto out_unlock; > > where wp_async is derived from ctx, not from cur. For example, a file-backed > VMA registered with wp_async enabled and UFFD_WP mode cannot be unregistered > through a uffd that does not have wp_async enabled. > > Rather than fix this and maintain this odd behavior, make unregistration > stricter by requiring VMAs to be unregistered through the same uffd they > were registered with. Additionally, reorder the WARN() checks to avoid > the aforementioned wp_async issue in the WARN()s. > > This change slightly modifies the ABI. It should not be backported to > -stable. Probably add that the expectation is that nobody really depends on this behavior, and that no such cases are known. > > While at it, correct the comment for the no userfaultfd case. This seems to > be a copy-paste artifact from the analogous userfaultfd_register() check. > > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization") Fixes should come before anything else in a series (Andrew even prefers a separate series for fixes vs. follow-up cleanups). > Signed-off-by: Tal Zussman > --- > fs/userfaultfd.c | 17 +++++++++++++---- > 1 file changed, 13 insertions(+), 4 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index 80c95c712266..10e8037f5216 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -1466,6 +1466,16 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, > VM_WARN_ON_ONCE(!!cur->vm_userfaultfd_ctx.ctx ^ > !!(cur->vm_flags & __VM_UFFD_FLAGS)); > > + /* > + * Check that this VMA isn't already owned by a different > + * userfaultfd. This provides for more strict behavior by > + * preventing a VMA registered with a userfaultfd from being > + * unregistered through a different userfaultfd. > + */ Probably we can shorted to: /* * Prevent unregistering through another userfaultfd than used for * registering. */ ? > + if (cur->vm_userfaultfd_ctx.ctx && > + cur->vm_userfaultfd_ctx.ctx != ctx) > + goto out_unlock; > + > /* > * Check not compatible vmas, not strictly required > * here as not compatible vmas cannot have an > @@ -1489,15 +1499,14 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx, > for_each_vma_range(vmi, vma, end) { > cond_resched(); > > - VM_WARN_ON_ONCE(!vma_can_userfault(vma, vma->vm_flags, wp_async)); > - > /* > - * Nothing to do: this vma is already registered into this > - * userfaultfd and with the right tracking mode too. > + * Nothing to do: this vma is not registered with userfaultfd. > */ Maybe /* VMA not registered with userfaultfd. */ The "skip" below is rather clear. :) > if (!vma->vm_userfaultfd_ctx.ctx) > goto skip; > > + VM_WARN_ON_ONCE(vma->vm_userfaultfd_ctx.ctx != ctx); > + VM_WARN_ON_ONCE(!vma_can_userfault(vma, vma->vm_flags, wp_async)); > WARN_ON(!(vma->vm_flags & VM_MAYWRITE)); > > if (vma->vm_start > start) > Acked-by: David Hildenbrand -- Cheers, David / dhildenb