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[2003:cb:c70f:6900:6c56:80f8:c14:6d2a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a073ccb8e1sm2665832f8f.59.2025.04.25.08.12.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 25 Apr 2025 08:12:38 -0700 (PDT) Message-ID: <23e2d207-58ac-49d3-b93e-4105a0624f9d@redhat.com> Date: Fri, 25 Apr 2025 17:12:37 +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 1/2] mm/userfaultfd: Fix uninitialized output field for -EAGAIN race To: Peter Xu , linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: Mike Rapoport , James Houghton , Suren Baghdasaryan , Axel Rasmussen , Andrew Morton , linux-stable , Andrea Arcangeli References: <20250424215729.194656-1-peterx@redhat.com> <20250424215729.194656-2-peterx@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: <20250424215729.194656-2-peterx@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 24.04.25 23:57, Peter Xu wrote: > While discussing some userfaultfd relevant issues recently, Andrea noticed > a potential ABI breakage with -EAGAIN on almost all userfaultfd ioctl()s. I guess we talk about e.g., "man UFFDIO_COPY" documentation: "The copy field is used by the kernel to return the number of bytes that was actually copied, or an error (a negated errno-style value). The copy field is output-only; it is not read by the UFFDIO_COPY operation." I assume -EINVAL/-ESRCH/-EFAULT are excluded from that rule, because there is no sense in user-space trying again on these errors either way. Well, there are cases where we would store -EFAULT, when we receive it from mfill_atomic_copy(). So if we store -EAGAIN to copy.copy it says "we didn't copy anything". (probably just storing 0 would have been better, but I am sure there was a reason to indicate negative errors in addition to returning an error) > > Quote from Andrea, explaining how -EAGAIN was processed, and how this > should fix it (taking example of UFFDIO_COPY ioctl): > > The "mmap_changing" and "stale pmd" conditions are already reported as > -EAGAIN written in the copy field, this does not change it. This change > removes the subnormal case that left copy.copy uninitialized and required > apps to explicitly set the copy field to get deterministic > behavior (which is a requirement contrary to the documentation in both > the manpage and source code). In turn there's no alteration to backwards > compatibility as result of this change because userland will find the > copy field consistently set to -EAGAIN, and not anymore sometime -EAGAIN > and sometime uninitialized. > > Even then the change only can make a difference to non cooperative users > of userfaultfd, so when UFFD_FEATURE_EVENT_* is enabled, which is not > true for the vast majority of apps using userfaultfd or this unintended > uninitialized field may have been noticed sooner. > > Meanwhile, since this bug existed for years, it also almost affects all > ioctl()s that was introduced later. Besides UFFDIO_ZEROPAGE, these also > get affected in the same way: > > - UFFDIO_CONTINUE > - UFFDIO_POISON > - UFFDIO_MOVE > > This patch should have fixed all of them. > > Fixes: df2cc96e7701 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races") > Fixes: f619147104c8 ("userfaultfd: add UFFDIO_CONTINUE ioctl") > Fixes: fc71884a5f59 ("mm: userfaultfd: add new UFFDIO_POISON ioctl") > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") > Cc: linux-stable > Cc: Mike Rapoport > Cc: Axel Rasmussen > Cc: Suren Baghdasaryan > Reported-by: Andrea Arcangeli > Suggested-by: Andrea Arcangeli > Signed-off-by: Peter Xu > --- > fs/userfaultfd.c | 28 ++++++++++++++++++++++------ > 1 file changed, 22 insertions(+), 6 deletions(-) > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > index d80f94346199..22f4bf956ba1 100644 > --- a/fs/userfaultfd.c > +++ b/fs/userfaultfd.c > @@ -1585,8 +1585,11 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx, > user_uffdio_copy = (struct uffdio_copy __user *) arg; > > ret = -EAGAIN; > - if (atomic_read(&ctx->mmap_changing)) > + if (unlikely(atomic_read(&ctx->mmap_changing))) { > + if (unlikely(put_user(ret, &user_uffdio_copy->copy))) > + return -EFAULT; > goto out; > + } Nit: It's weird that we do "return -EFAULT" in one case, in the other we do "goto out;" which ends up doing a "return ret" ... Maybe to keep it consistent: ret = -EAGAIN; if (unlikely(atomic_read(&ctx->mmap_changing))) { if (unlikely(put_user(ret, &user_uffdio_copy->copy))) ret = -EFAULT; goto out; } In all of these functions, we should probably just get rid of the "goto out" and just return directly. We have a weird mixture of "goto out;" and return; ... a different cleanup. Reviewed-by: David Hildenbrand -- Cheers, David / dhildenb