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[69.131.150.190]) by smtp.gmail.com with ESMTPSA id 00721157ae682-7bd258a2eafsm16410797b3.29.2026.04.29.08.06.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Apr 2026 08:06:19 -0700 (PDT) Message-ID: Date: Wed, 29 Apr 2026 11:06:13 -0400 Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Copy-on-read for untrusted image mounts, and differentiating between metadata and user data writes. From: Demi Marie Obenour To: Theodore Tso , Zw Tang Cc: Andreas Dilger , libaokun@linux.alibaba.com, jack@suse.cz, ojaswin@linux.ibm.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com References: <20260421122059.GA86221@macsyma.local> <4e76eb68-862d-4b9f-8242-e6aced2704ee@gmail.com> Content-Language: en-US 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: <4e76eb68-862d-4b9f-8242-e6aced2704ee@gmail.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="------------FeJLxpq4D0qzPN6ajjdfRHpw" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --------------FeJLxpq4D0qzPN6ajjdfRHpw Content-Type: multipart/mixed; boundary="------------uVtZiUWuP0wseXO4L1R4xZvl"; protected-headers="v1" Message-ID: Date: Wed, 29 Apr 2026 11:06:13 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Copy-on-read for untrusted image mounts, and differentiating between metadata and user data writes. From: Demi Marie Obenour To: Theodore Tso , Zw Tang Cc: Andreas Dilger , libaokun@linux.alibaba.com, jack@suse.cz, ojaswin@linux.ibm.com, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com References: <20260421122059.GA86221@macsyma.local> <4e76eb68-862d-4b9f-8242-e6aced2704ee@gmail.com> Content-Language: en-US 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: <4e76eb68-862d-4b9f-8242-e6aced2704ee@gmail.com> --------------uVtZiUWuP0wseXO4L1R4xZvl Content-Type: multipart/mixed; boundary="------------35CRNHw0ylgbaSLfzDu87zmy" --------------35CRNHw0ylgbaSLfzDu87zmy Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On 4/25/26 14:00, Demi Marie Obenour wrote: > On 4/21/26 08:20, Theodore Tso wrote: >> On Tue, Apr 21, 2026 at 07:32:43PM +0800, Zw Tang wrote: >>> This looks like an ext4 inline-data boundary/state inconsistency trig= gered >>> while writing to an ext4 image crafted by syzkaller. The later >>> KASAN: slab-use-after-free in rwsem_down_write_slowpath() appears to = be a >>> secondary effect after the primary ext4 BUG, likely during teardown/u= nlink >>> after the filesystem failure. >> >> Writing to a mounted image is not something that we consider a valid >> threat model. If you can write to a mounted image, there are a >> zillion different ways that you can creash the kernel, or you can >> create a setuid shell, etc. >> >> The upstream syzkaller bot makes sure that CONFIG_BLK_DEV_WRITE_MOUNTE= D >> is not defined to avoid syzkaller noise. >=20 > CONFIG_BLK_DEV_WRITE_MOUNTED only blocks writing via the specific block= > device that is mounted. It doesn't block writing via other methods. > If I recall correctly, its purpose was to prevent writing to the > buffer cache used by the filesystem driver. >=20 > Changing block devices that are mounted is also reachable via USB. > Yes, some distros may disable automount, but users who have stuff to > get done will mount USB devices anyway. Telling users "don't do this" > very rarely works in practice. So this gave me an idea that might work in practice, without requiring any additional work from the ext4 (or XFS) developers. My understanding is that: 1. e2fsck *is* intended to be secure against malicious filesystem images (though not TOCTOU). 2. Mounting with nosuid,noexec,nodev,nosymfollow can mitigate VFS-level attacks. This means that the following should be a safe way to mount an untrusted ext4 filesystem: 1. Copy it to trusted storage. 2. Run 'e2fsck -f -n' (or 'e2fsck -f -p') on the image. 3. Mount it with nosuid,noexec,nodev,nosymfollow. The first step protects against TOCTOU, and the second protects against metadata parsing attacks. It's possible to optimize this by using a virtual block device that: 1. When data is read for the first time, copies it from the untrusted device to the trusted device. Subsequent reads come from the trusted device. 2. When data is written, it is written to both the untrusted and trusted devices. However, if one is mounting read-only, one can actually go further than that for ext4. ext4 doesn't care about file contents, only metadata. So TOCTOU attacks on file contents can't affect it. Furthermore, e2fsck must read all metadata in order to do its job. After all, the safety of step 3 in the above procedure depends on step 2 doing exactly that. If the filesystem is mounted with 'ro,noatime', then (IIUC) the only change that the ext4 driver will need to make to the filesystem is journal replay. But e2fsck has to do that to check the integrity of the filesystem. This means that copy-on-read is only necessary while e2fsck is running. Afterwards, it's no longer necessary to copy newly-accessed data to the trusted device. Erroring any writes to data that was not already read will ensure that further metadata (that needs TOCTOU protection) is not written out. This doesn't protect against TOCTOU attacks on applications, but for at least some backup workloads that is not an issue. With minimal cooperation from the filesystem, one can do even better. If the device knew what was metadata and what was file contents, it could do better by only storing metadata on trusted storage, while file contents are allowed to be written to untrusted storage. his has many applications in the server world! It's quite possible that one wants to treat metadata and file contents differently. For instance, one might want to put the metadata on a fast NVMe drive (or a RAID 1 of two or more such drives), while the file contents are on a RAID 6 on HDDs. This keeps metadata access (lots of random reads and writes) fast, while bulk data access (likely sequential and performance-insensitive) can go on cheap bulk storage. --=20 Sincerely, Demi Marie Obenour (she/her/hers) --------------35CRNHw0ylgbaSLfzDu87zmy Content-Type: application/pgp-keys; name="OpenPGP_0xB288B55FFF9C22C1.asc" Content-Disposition: attachment; filename="OpenPGP_0xB288B55FFF9C22C1.asc" Content-Description: OpenPGP public key Content-Transfer-Encoding: quoted-printable -----BEGIN PGP PUBLIC KEY BLOCK----- xsFNBFp+A0oBEADffj6anl9/BHhUSxGTICeVl2tob7hPDdhHNgPR4C8xlYt5q49y B+l2nipdaq+4Gk6FZfqC825TKl7eRpUjMriwle4r3R0ydSIGcy4M6eb0IcxmuPYf bWpr/si88QKgyGSVZ7GeNW1UnzTdhYHuFlk8dBSmB1fzhEYEk0RcJqg4AKoq6/3/ UorR+FaSuVwT7rqzGrTlscnTDlPWgRzrQ3jssesI7sZLm82E3pJSgaUoCdCOlL7M MPCJwI8JpPlBedRpe9tfVyfu3euTPLPxwcV3L/cfWPGSL4PofBtB8NUU6QwYiQ9H zx4xOyn67zW73/G0Q2vPPRst8LBDqlxLjbtx/WLR6h3nBc3eyuZ+q62HS1pJ5EvU T1vjyJ1ySrqtUXWQ4XlZyoEFUfpJxJoN0A9HCxmHGVckzTRl5FMWo8TCniHynNXs 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Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="OpenPGP_signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEopQtqVJW1aeuo9/sszaHOrMp8lMFAmnyHmYACgkQszaHOrMp 8lOkJA/9GC7yDPebFfVTvtAsa0wQ7tGBVnhNx+fJWPRytgJok0IDaWsGSvjuONR8 CLzf+s+97GuBJSSvF2jzVj91axslalkXbR3RuoriczyUdtCKMYg+lw67xVAYZI3S 6uXI4MlKWSTZxJgFIuMXebafAJqxnAXkwQJ5kfECA0Cxm3xe2o/H+USm1TMGRsKy pKO9a8YsX5G1+Z00hYGDTf2wdVK0fLfO1te+LUvLOc82TDSOIz97RaFmhVaAX6Z9 kFCZC+PNnx0r46FZZUVNIRDhfycEXMpuX+Oy0GoFCTFI4niJIs+XyOJ8FImh37Hj hJfPuKKU2FJC3hiVmFZQruJ4HfqXcB1XTLlrsCtZwQzaTe/2Ei6QMrFNExygYxT6 CnQGHf/K7/hMMyvARWN14gftG26IpMblJephpcqprjZGVszI2G5afzlb3GLqZIQB EiTH1K2JnwI4IduvPy5QjKjptYzo4nbaQA9lNNvwkXaVpsj1cvnnBoluhP30L3kk rCYodeTHdqHeEYByUxZ4OvBXB/YdteK6MosDRiOqQVRNVqJmFhbMNkm8oAYalq92 +nqNhO6msTGxs9+1A1DhCwWCNjw2312HvvDZTehkQIQDDTxiW54Wl7mda1b8HrQZ 4y60HB3yb6K50dez+lTjbviRvkusHhyWIzl1ulQqYVYakkDrV+I= =N/RY -----END PGP SIGNATURE----- --------------FeJLxpq4D0qzPN6ajjdfRHpw--