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[2003:d8:2f31:700:3851:c66a:b6b9:3490]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568b08c7csm13601911f8f.50.2025.06.17.02.13.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 17 Jun 2025 02:13:46 -0700 (PDT) Message-ID: Date: Tue, 17 Jun 2025 11:13:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] hugetlb: block hugetlb file creation if hugetlb is not set up To: Jann Horn , Andrew Morton Cc: Muchun Song , Oscar Salvador , linux-mm@kvack.org, Lorenzo Stoakes References: <20250528-hugetlb-nerf-v1-1-a404ca33e819@google.com> <20250602204107.177e2fdf2209b0926b5ce28e@linux-foundation.org> 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: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: cbVPw0xXt4kGxzZjqrk_MljxulAE5Voz_7A1Zth7KHY_1750151627 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam01 X-Stat-Signature: fw43qwxi75d9s5rw9j6whnhx8hdeuy1q X-Rspamd-Queue-Id: 10B20120008 X-Rspam-User: X-HE-Tag: 1750151630-95766 X-HE-Meta: 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 mhRBosja 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 03.06.25 06:29, Jann Horn wrote: > On Tue, Jun 3, 2025 at 5:41 AM Andrew Morton wrote: >> On Wed, 28 May 2025 19:51:29 +0200 Jann Horn wrote: >>> Many distro kernels enable hugetlb support, but most systems running >>> those kernels never actually allocate hugepages or enable hugetlb >>> overcommit. >>> >>> On such systems, hugetlb is unusable for any legitimate usecase, but it >>> is still possible to exercise a lot of hugetlb-specific code by creating >>> MAP_HUGETLB|MAP_NORESERVE VMAs - for example, it is still possible to >>> create page tables shared across processes. >>> >>> This is exposed through the mmap() syscall, with no privileges required, >>> so from a security perspective, this is interesting attack surface. >>> >>> Lock it down by completely denying creation of hugetlb files if no huge >>> pages for the hstate could be allocated without administratively >>> changing huge page limits. >> >> So this is a non-backward-compatible change? > > Yes, this change changes kernel behavior that is userspace-visible, > and causes syscalls to return errors where they worked before. > >> If any userspace is affected it's probably either stupid or evil, but I >> do wonder if there are legit cases for doing this, such as "I don't >> know if there are any hugepages configured, but I'll try this anyway >> and figure out what to do later on". And maybe there are other legit >> cases! > > Right. I think an affected case would be if userspace tries to detect > whether the kernel supports hugepages by creating a MAP_NORESERVE > mapping or huge memfd, and if that works, twiddles sysfs knobs to > actually allocate hugepages or shows a specific error message. Such a > program might end up wrongly assuming that the kernel does not support > hugepages. My understanding is that hugepages are normally > administratively configured so that they can be allocated early during > boot without having to worry about RAM fragmentation, in which case > this probably wouldn't happen, but it's not like I actually have a > good understanding of how typical hugetlb users work. > > Another affected case would be if userspace confirms that the kernel > supports hugetlb through sysfs or such, then creates a MAP_NORESERVE > hugetlb and asserts that this must work because MAP_NORESERVE more or > less can't fail, and crashes with an assertion failure or such. > > My understanding is that the combination of MAP_HUGETLB and > MAP_NORESERVE is somewhat rare in the first place; searching debian > codesearch for both flags on the same line, I basically only get one > hit in the "gridtools" package, though there might well be other cases > where the flags are set on separate lines. memfd_create(MFD_HUGETLB) > seems to be more common. QEMU can trigger this, and there might be corner-case use cases where you setup a virtio-mem device (to hotplug memory later) when staring the VM, but actually allocate the huge pages only when wanting to provide them to the VM. It's not that common, because usually you back all your VM through huge pages, not just hotplugged memory. But it's definitely possible ... -- Cheers, David / dhildenb