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[96.60.249.169]) by smtp.gmail.com with ESMTPSA id 3f1490d57ef6-ea3cb61dff5sm126811276.9.2025.09.10.13.35.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Sep 2025 13:35:35 -0700 (PDT) Message-ID: <37d4dc51-4ee1-4ccf-8d37-2272988b9361@gmail.com> Date: Wed, 10 Sep 2025 16:35:30 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Sandboxing strategy To: Alyssa Ross References: <87o6ritk9q.fsf@alyssa.is> Content-Language: en-US From: Demi Marie Obenour Autocrypt: addr=demiobenour@gmail.com; keydata= xsFNBFp+A0oBEADffj6anl9/BHhUSxGTICeVl2tob7hPDdhHNgPR4C8xlYt5q49yB+l2nipd aq+4Gk6FZfqC825TKl7eRpUjMriwle4r3R0ydSIGcy4M6eb0IcxmuPYfbWpr/si88QKgyGSV Z7GeNW1UnzTdhYHuFlk8dBSmB1fzhEYEk0RcJqg4AKoq6/3/UorR+FaSuVwT7rqzGrTlscnT DlPWgRzrQ3jssesI7sZLm82E3pJSgaUoCdCOlL7MMPCJwI8JpPlBedRpe9tfVyfu3euTPLPx wcV3L/cfWPGSL4PofBtB8NUU6QwYiQ9Hzx4xOyn67zW73/G0Q2vPPRst8LBDqlxLjbtx/WLR 6h3nBc3eyuZ+q62HS1pJ5EvUT1vjyJ1ySrqtUXWQ4XlZyoEFUfpJxJoN0A9HCxmHGVckzTRl 5FMWo8TCniHynNXsBtDQbabt7aNEOaAJdE7to0AH3T/Bvwzcp0ZJtBk0EM6YeMLtotUut7h2 Bkg1b//r6bTBswMBXVJ5H44Qf0+eKeUg7whSC9qpYOzzrm7+0r9F5u3qF8ZTx55TJc2g656C 9a1P1MYVysLvkLvS4H+crmxA/i08Tc1h+x9RRvqba4lSzZ6/Tmt60DPM5Sc4R0nSm9BBff0N m0bSNRS8InXdO1Aq3362QKX2NOwcL5YaStwODNyZUqF7izjK4QARAQABzTxEZW1pIE1hcmll IE9iZW5vdXIgKGxvdmVyIG9mIGNvZGluZykgPGRlbWlvYmVub3VyQGdtYWlsLmNvbT7CwXgE EwECACIFAlp+A0oCGwMGCwkIBwMCBhUIAgkKCwQWAgMBAh4BAheAAAoJELKItV//nCLBhr8Q AK/xrb4wyi71xII2hkFBpT59ObLN+32FQT7R3lbZRjVFjc6yMUjOb1H/hJVxx+yo5gsSj5LS 9AwggioUSrcUKldfA/PKKai2mzTlUDxTcF3vKx6iMXKA6AqwAw4B57ZEJoMM6egm57TV19kz PMc879NV2nc6+elaKl+/kbVeD3qvBuEwsTe2Do3HAAdrfUG/j9erwIk6gha/Hp9yZlCnPTX+ VK+xifQqt8RtMqS5R/S8z0msJMI/ajNU03kFjOpqrYziv6OZLJ5cuKb3bZU5aoaRQRDzkFIR 6aqtFLTohTo20QywXwRa39uFaOT/0YMpNyel0kdOszFOykTEGI2u+kja35g9TkH90kkBTG+a EWttIht0Hy6YFmwjcAxisSakBuHnHuMSOiyRQLu43ej2+mDWgItLZ48Mu0C3IG1seeQDjEYP tqvyZ6bGkf2Vj+L6wLoLLIhRZxQOedqArIk/Sb2SzQYuxN44IDRt+3ZcDqsPppoKcxSyd1Ny 2tpvjYJXlfKmOYLhTWs8nwlAlSHX/c/jz/ywwf7eSvGknToo1Y0VpRtoxMaKW1nvH0OeCSVJ itfRP7YbiRVc2aNqWPCSgtqHAuVraBRbAFLKh9d2rKFB3BmynTUpc1BQLJP8+D5oNyb8Ts4x Xd3iV/uD8JLGJfYZIR7oGWFLP4uZ3tkneDfYzsFNBFp+A0oBEAC9ynZI9LU+uJkMeEJeJyQ/ 8VFkCJQPQZEsIGzOTlPnwvVna0AS86n2Z+rK7R/usYs5iJCZ55/JISWd8xD57ue0eB47bcJv VqGlObI2DEG8TwaW0O0duRhDgzMEL4t1KdRAepIESBEA/iPpI4gfUbVEIEQuqdqQyO4GAe+M kD0Hy5JH/0qgFmbaSegNTdQg5iqYjRZ3ttiswalql1/iSyv1WYeC1OAs+2BLOAT2NEggSiVO txEfgewsQtCWi8H1SoirakIfo45Hz0tk/Ad9ZWh2PvOGt97Ka85o4TLJxgJJqGEnqcFUZnJJ riwoaRIS8N2C8/nEM53jb1sH0gYddMU3QxY7dYNLIUrRKQeNkF30dK7V6JRH7pleRlf+wQcN fRAIUrNlatj9TxwivQrKnC9aIFFHEy/0mAgtrQShcMRmMgVlRoOA5B8RTulRLCmkafvwuhs6 dCxN0GNAORIVVFxjx9Vn7OqYPgwiofZ6SbEl0hgPyWBQvE85klFLZLoj7p+joDY1XNQztmfA rnJ9x+YV4igjWImINAZSlmEcYtd+xy3Li/8oeYDAqrsnrOjb+WvGhCykJk4urBog2LNtcyCj kTs7F+WeXGUo0NDhbd3Z6AyFfqeF7uJ3D5hlpX2nI9no/ugPrrTVoVZAgrrnNz0iZG2DVx46 x913pVKHl5mlYQARAQABwsFfBBgBAgAJBQJafgNKAhsMAAoJELKItV//nCLBwNIP/AiIHE8b oIqReFQyaMzxq6lE4YZCZNj65B/nkDOvodSiwfwjjVVE2V3iEzxMHbgyTCGA67+Bo/d5aQGj gn0TPtsGzelyQHipaUzEyrsceUGWYoKXYyVWKEfyh0cDfnd9diAm3VeNqchtcMpoehETH8fr RHnJdBcjf112PzQSdKC6kqU0Q196c4Vp5HDOQfNiDnTf7gZSj0BraHOByy9LEDCLhQiCmr+2 E0rW4tBtDAn2HkT9uf32ZGqJCn1O+2uVfFhGu6vPE5qkqrbSE8TG+03H8ecU2q50zgHWPdHM OBvy3EhzfAh2VmOSTcRK+tSUe/u3wdLRDPwv/DTzGI36Kgky9MsDC5gpIwNbOJP2G/q1wT1o Gkw4IXfWv2ufWiXqJ+k7HEi2N1sree7Dy9KBCqb+ca1vFhYPDJfhP75I/VnzHVssZ/rYZ9+5 1yDoUABoNdJNSGUYl+Yh9Pw9pE3Kt4EFzUlFZWbE4xKL/NPno+z4J9aWemLLszcYz/u3XnbO vUSQHSrmfOzX3cV4yfmjM5lewgSstoxGyTx2M8enslgdXhPthZlDnTnOT+C+OTsh8+m5tos8 HQjaPM01MKBiAqdPgksm1wu2DrrwUi6ChRVTUBcj6+/9IJ81H2P2gJk3Ls3AVIxIffLoY34E +MYSfkEjBz0E8CLOcAw7JIwAaeBT In-Reply-To: <87o6ritk9q.fsf@alyssa.is> Content-Type: multipart/signed; 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boundary="------------GQQHhhkUtV274Ewa3s0zGHoN"; protected-headers="v1" From: Demi Marie Obenour To: Alyssa Ross Cc: Spectrum OS Development Message-ID: <37d4dc51-4ee1-4ccf-8d37-2272988b9361@gmail.com> Subject: Re: Sandboxing strategy References: <87o6ritk9q.fsf@alyssa.is> In-Reply-To: <87o6ritk9q.fsf@alyssa.is> Autocrypt-Gossip: addr=hi@alyssa.is; keydata= xsFNBFpSgoYBEAC4xkCYidG2JlRWulUkTWcx0pHFDf3oSbb6Q872Kb3iDChWgluNVz43hva1 3xfDo9foV0GoyfGl/ycSCkXX5hlQr7ir/5FN38E7H/yY6tH8+l68iDgIOcb1qY0OYaxyg+Lz WesfFQedrmwNTbF4L1BtWzrTR5PflDdhDo5VWSguHGJFSclchcr/6UmMb/gOUN+2ElBC2TE2 EKY099phZ6DJZ2aZCsclwKIdCpZzXlEmXPAeaH5om6xo90JYv5+sFji40R0Plqec3WC+jTxy lGca6IbPdOminuUF+GvsR86eVsgh/0XNK7/zus7gyc4PuMUA1rCoeHcWOBDPgmelgCQyJGXd /bXeKuUsGoge58uc7/YNvOh1vfpD3AaEMqAyXfmmUwBnIicml74+2eOpH3Oljfs01g+DhkOB MtpVSZSgaIDvP0WG6cbAxImoUasnmNxEDNskfVmI8bsajPW9bt4z5hiP5Q9G3vE0D5HcIFdM adOz81PpOwNiUXcjtYV1PWZQ56jbSTOf8EBvsB71WwB+XgVWcPzIlY8hAykiHIO87oV3o71U JTAn1Foj7mjSADnY0deleOmar/K5jrK3wvKKM1XlB7PXcGBdkorJC+cbxVsw0ADzMw0c7bVc wEE7OFvHjQiIK1lO+lb1cvGBBY3IZxjsjZdA/VsFHFdAeYlzNQARAQABzRpBbHlzc2EgUm9z cyA8aGlAYWx5c3NhLmlzPsLBlwQTAQgAQQIbAwULCQgHAgYVCAkKCwIEFgIDAQIeAQIXgAIZ ARYhBHVzVtd5u7iIdz5BXnNszfnvUb2XBQJoh9HmBQkOtoJgAAoJEHNszfnvUb2XFxAP/jF7 Rxuspytz7VwOA/y5WQe/FdPNRvyRUzj66ZzfuaG++md14LWcrvbb2hHLdS5ePdKoOaqC3xhK Ncy/5RVd4pD5N4AY7O5228JKDO8GGWEl3rupOCmYQL9P8GpsdGt2xkFqbu4HrFzMTzl6nvKN GtvhBSCkAW3v7wTwPf2rkjX8f+clmRZn4YLfojaBCT/ZFjmJkQSWZ+2rUZOxjgZgckLkplu8 LVILVdaHNLkOrxbKQLuuVyEkN+/YrshyJP4a7Z/Tnw+3eZcaLuLOxBHpWOAAgJZ8kL+V7BxK DlTywv/OOr0CoNi9zjH3VgRiJ1961K/DocjA50pWBilMUs4jGp9omhkAOwdNV2hi+Zj/GXaD kgvBlE313DGWzgW0RauL8j5/GULgphwm5UOHXb/5yM9pRw87kfQ/+zQuHpguoXSGkUv0kSID 88kUGlNxy1kkTsSYG2UTKLmLDPuUGOUDGaNUzD+J49MogSLTmvHUIYNs0jFmpLyhVoTtSi/I VQQpHyk9ucAZd2OLxChm3RBJhAbmGw3rhUxx6B8SHwcBcJDttGyYF2GZj9AYAGlU/Fm1Itoi NwJD0vQnSZ//25l+XJehpeeixsqydj00gksHWEFaIEgCYyC6JfmGS4HtuUcv76vc2vLCv5aH hJVHHKGs+kiKJ2+r7bCNwvW0doVePdByzjgEZyd/yxIKKwYBBAGXVQEFAQEHQCVxoiHOlsEo NDKGCbxg4nL3E1CV0MRQCU1hPowd77h3AwEIB8LBfAQYAQoAJgIbDBYhBHVzVtd5u7iIdz5B XnNszfnvUb2XBQJoh9HXBQkB4YUCAAoJEHNszfnvUb2XgvAP+gIIKR3KatmwuUcla1RN6tUc 0Ae9eqS1cbSEDj8a14fnvUHtlCEO5g4m2PvPn7L2o7047ppl5100+zGMocXWslnDFJJVeAOp xzlhZ48DFMxkZkjtLY2fRrj1tOtzrTRikV0/UsaVs9tMPMEsVu/FH0exMoNELcqEVUgcRRpd XYXC9zgDUcM4+JRH8hDOiC51JRvdkXcokAXphuCY3J996i8Ph+kg6xMqscePscvvltR68jcT HHDOiOQtzjJRKQ6BHUzYdIRixcq210sabfZGJVT+7BfHI5qiUwfoG3DXSJygviN3I/EfnSbD 6T0Ccv/G6wQPHO4Vyn+egrMOU/HUrCKuq/b0rYvI8pTtRWAxIHkFZoKVB8I8Rl8+riGiEUAh Ja0bU4iOg7vR5Bm8Gt/tR6KjClyBI5NN1M1haFgVXqZ3dsQDpBjivbDHbUsAXVsd00y0B4Ir 8muvII9eJOwbTyqcG9RJbflquPp+FPlgiy9pYj52PB0NwzLYqX0fOWU8IqQF3bWtdJIb4Zat XnfqYtJ/wCaJTU43N3+/TIdqNA4G3WbyRYsCnz/kkf1ARmxoq3r29dOl8OnNHEvB1+hBlNBk d+cLpsYdTxa63OpbNDpm/unX1sEpOYy04dL5Y8B1Xbh+rQFNySq4Ani2dl5N1OsrjgzZ+wkl k2wOTWzl7wFH --------------GQQHhhkUtV274Ewa3s0zGHoN Content-Type: multipart/mixed; boundary="------------4ecWuz3gA4zFoD4IQZ473NCC" --------------4ecWuz3gA4zFoD4IQZ473NCC Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 9/10/25 11:11, Alyssa Ross wrote: > Demi Marie Obenour writes: >=20 >> I was thinking about how to sandbox the various per-VM daemons >> and came up with the following strategy: >> >> - Each VM gets its own PID and mount namespace and set of user IDs. >=20 > Didn't you say to me we couldn't do PID namespaces without support from= > s6? I was mistaken about this. Without direct support in s6, there is no way to avoid having a persistent process outside the PID namespace as s6's direct child, but that is harmless. >> - Mount namespace includes /proc, /sys, /dev, and the host rootfs. >> >> - Each service gets its own /tmp and /dev/shm if they are needed at al= l. >=20 > Just a question: if we put services into cgroups, does use of tmpfs get= > charged to the appropriate cgroup? It definitely should, especially if the tmpfs is mounted from inside the cgroup. Whether it actually does I don't know. >> - virtiofsd gets r/w access to the VM private storage. >> >> - IPC namespaces are irrelevant because the kernel is >> built without System V IPC or POSIX message queues. >> >> - Sending signals between services in the namespace is blocked >> by Landlock. Landlock also blocks ptrace() and other nastiness, >> as well as communication via abstract AF_UNIX sockets. >> >> - Since AF_UNIX abstract sockets between services are blocked by >> Landlock and Spectrum builds without IP or even Ethernet on the >> host there is no need for network namespacing. >=20 > It doesn't currently, just to be clear. (I'm still putting off using a= > custom kernel config on the host until we have better tooling for > keeping up with Nixpkgs.) Makes sense. >> - The sandbox manager is PID 1 in the VM's PID namespace. >> When s6 tells it to shut down, it tries to gracefully shut >> down the VM. After a timeout or once the VM has shut down, >> it exits, and Linux automatically kills all the processes >> and cleans up the mount namespace. >> >> - The sandbox manager uses prctl(PR_SET_PDEATHSIG) to ensure it >> dies if the parent s6 process dies. This requires s6 to provide >> its own PID to avoid races, but that is easy to implement. >> >> All of this behavior will be hard-coded into C and Rust source code, >> so it will be vastly simpler than a generic program that must support >> many use-cases. >=20 > This all sounds fine, BUT there are a couple of important things to bea= r > in mind: >=20 > =E2=80=A2 This needs to be maintainable. I don't know how much code t= his is > going to be our how complex it's going to be, but that this will be > totally custom does make me a bit concerned. This should not be too difficult. It's the same system calls used by container managers, so if there is a problem it should be possible to get help fairly easily. bubblewrap=20 > =E2=80=A2 These services are part of our TCB anyway. Sandboxing only = gets us > defense in depth. With that in mind, it's basically never going to > be worth adding sandboxing if it adds any amount of attack surface. > One example of that would be user namespaces. They've been a > consistent source of kernel security issues, and it might be better > to turn them off entirely than to use them for sandboxing stuff > that's trusted anyway. Sandboxing virtiofsd is going to be really annoying and will definitely come at a performance cost. The most efficient way to use virtiofsd is to give it CAP_DAC_READ_SEARCH in the initial user namespace and delegate _all_ access control to it. This allows virtiofs to use open_by_handle_at() for all filesystem access. Unfortunately, this also allows virtiofsd to open any file on the filesystem, ignoring all discretionary access control checks. I don't think Landlock would work either. SELinux or SMACK might work, but using them is significantly more complicated. If one wants to sandbox virtiofsd, one either needs to use --cache=3Dnever or run into an effective resource leak (https://gitlab.com/virtio-fs/virtiofsd/-/issues/194). My hope is that in the future the problem will be solved by DAX and an in-kernel shrinker that is aware of the host resources it is using. 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