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(p200300cbc70617006b8127cbc9e2a09a.dip0.t-ipconnect.de. [2003:cb:c706:1700:6b81:27cb:c9e2:a09a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42ba6397dcdsm16626955e9.6.2024.08.28.03.05.57 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 Aug 2024 03:05:57 -0700 (PDT) Message-ID: Date: Wed, 28 Aug 2024 12:05:56 +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] prctl: allow prctl_set_mm_exe_file without unmapping old exe To: Theodore Dubois , linux-kernel@vger.kernel.org Cc: Ryan Houdek , "Guilherme G. Piccoli" , "Eric W. Biederman" , Kees Cook References: <20240827215930.24703-1-tblodt@icloud.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: <20240827215930.24703-1-tblodt@icloud.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 27.08.24 23:59, Theodore Dubois wrote: > As far as I can tell, the original purpose of this check was simply as > the easiest way to work with a quirk of /proc/self/exe at the time. From > the original patch[1]: > > Note it allows to change /proc/$pid/exe iif there > are no VM_EXECUTABLE vmas present for current process, > simply because this feature is a special to C/R > and mm::num_exe_file_vmas become meaningless after > that. > > num_exe_file_vmas was created to preserve a quirk of the original > /proc/self/exe implementation: if you unmapped all executable VMAs, > /proc/self/exe would disappear (because it worked by scanning the > address space for the first executable VMA.) Keeping the quirk after > switching to just saving the executable on the mm worked by keeping a > count of executable VMAs in num_exe_file_vmas, and zeroing exe_file when > it reached zero. You can probably see how it would have been annoying to > handle both num_exe_file_vmas and this prctl intending to change > exe_file, and it's easier to only allow changing exe_file after > num_exe_file_vmas has already gone to 0 and made exe_file null. > > However, num_exe_file_vmas no longer exists[2]. This quirk was taken out > because it would save a bit in the vma flags, and it seems clear by now > that nobody was relying on it. These days you can simply update exe_file > with no interference. > > Recently a use case for this prctl has come up outside of > checkpoint/restore, namely binfmt_misc based emulators such as FEX[3]. > Any program that uses /proc/self/exe will, of course, expect it to point > to its own executable. But when executed through binfmt_misc, it will be > the emulator, resulting in compatibility issues. Emulators currently > have to attempting to intercept syscalls targeting /proc/self/exe to > redirect the path, and this is not possible in the general case > considering how flexible path resolution is. For more detail on this see > [3]. > > The above seems to me like a solid case for simply dropping the check. Interestingly, the man page states: "You can even type /proc/pid/exe to run another copy of the same executable that is being run by process pid." Is that still true (with that binfmt_misc magic) once we change /proc/self/exe? Or does it with changing /proc/self/exe to point at the non-emulator exe even work as expected regarding this documentation? It's a good question what will change if processes start setting random other stuff while they are still executing part of the original binary. commit 4229fb1dc6843c49a14bb098719f8a696cdc44f8 Author: Konstantin Khlebnikov Date: Wed Jul 11 14:02:11 2012 -0700 c/r: prctl: less paranoid prctl_set_mm_exe_file() temporarily switch to checking that not other files besides the executable are still mapped. I agree that b32dfe3771 reads like this check was added primarily " because this feature is a special to C/R and mm::num_exe_file_vmas become meaningless after that" Reading mailing list discussions, there was a discussion regarding security risks [1] with the conclusion being "We must not trust /proc/pid/exe in anyway. An attacker can always execute another binary without calling execve()." [2]. So with that in mind, no objection. Clarifying which effect this has on what's stated in the man page would be interesting. [1] https://lore.kernel.org/lkml/CAFLxGvxEDs=RG7tX+j6XEUx2+wEvuCGipUzh2vSp3rj15Rq6zA@mail.gmail.com/ [2] https://lore.kernel.org/lkml/CAFLxGvyCRAq6t8_ni+VFUVpOGJ4-iz0i=PRFEFpVJ+ZaPEb3-g@mail.gmail.com/ -- Cheers, David / dhildenb