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boundary="------------0BQDKKn9cP29OPNRK45Ha9VW"; protected-headers="v1"; hp="clear" Message-ID: <8b45a161-101f-4f28-9b7a-c7c8b8a29f90@gmail.com> Date: Wed, 4 Nov 2026 19:56:11 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird From: Demi Marie Obenour Subject: MAP_FIXED leaving holes on failure is very difficult to recover from and can even be a security hole To: Linux kernel mailing list , linux-mm@kvack.org 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 Cc: Bert Karwatzki , Jeff Xu , Jiri Olsa , Kees Cook , Lorenzo Stoakes , Mark Brown , Matthew Wilcox , "Paul E. McKenney" , Paul Moore , Sidhartha Kumar , Suren Baghdasaryan , Vlastimil Babka , Andrew Morton , "Liam R. Howlett" --------------0BQDKKn9cP29OPNRK45Ha9VW Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Right now, a failed MAP_FIXED mmap() can leave a hole in the address space. Another mmap() can then enter this hole. This was fixed in the past, but the fix was undone in 4f87153e82c4 ("mm: change failure of MAP_FIXED to restoring the gap on fa= ilure"). This is really nasty, and can create serious problems for userspace. A simple example is MAP_FIXED over a part of a KVM memslot to expose memory to a guest. If that leaves a hole, the result is catastrophic: Subsequent mmap()s (perhaps by malloc()) can get put into that address range. KVM will give the guest access to whatever happens to be put there, and one must assume the guest can turn that into a VM escape. Other situations don't result in immediate disaster, but are still really hard to recover from. One can use MAP_FIXED_NOREPLACE to refill the hole if nobody else has filled it, but if someone has, there's no hope of making the original mapping (that the MAP_FIXED had clobbered) contiguous again. One must either figure out how to handle a non-contiguous mapping, rebuild the mapping elsewhere, or crash. In my case, I can work around the problem by using a single memslot per file descriptor, but I suspect there are other use-cases that aren't so lucky. Yet others may simply not have realized this was possible; there's even a Stack Overflow answer [1] that claims that the map is replaced atomically. I only found out about it from Claude Opus 5.5. I asked it to write a test suite, and it pointed out that a failed MAP_FIXED mmap() can leave a hole. If I hadn't asked, I would have shipped a security vulnerability. Should the behavior be changed again, so that mmap() with MAP_FIXED is atomic? That would provide a UAPI that actually makes sense and can be used without horribly ugly workarounds. [1]: https://stackoverflow.com/a/28576674 --=20 Sincerely, Demi Marie Obenour (she/her/hers) --------------0BQDKKn9cP29OPNRK45Ha9VW-- --------------xVK0XLy9X0vduJNb936CFAdr Content-Type: application/pgp-signature; name="OpenPGP_signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="OpenPGP_signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEopQtqVJW1aeuo9/sszaHOrMp8lMFAmrr1CwACgkQszaHOrMp 8lOVcg//SKjfMVWdZ0NQUQB8SGUy1DUvasvgNKbECkMRVXoKID1J91XoXk0hW6a9 U7CFyJjrLcIkxHBXo3i+AHcz/9R7BD3M9XwnQAk3ZcN7XdpgzjGNXDixXftPcay1 lFuZfI3btf9V+AADMiP2EwrwseYFFciXKxaSvFlmV8it5+8xbffiCyyjbZ5RFZ/g s8x1bJ/XzpZCPv9NZyn4lHWBcbJ2eN1N6bgblOaKHLLN7krBDF2fQWYlFXmOieWT weTFXZxKnH/NpwVlJfL2jeCkYXUdkdDHOSWJm0taFbfV2Wct1mgXXewrjSrDTgyI 1G9iWh9zr6+Qu8jtz8/XPYltG3/akStk61ncQEo4PUGg7hkEG3haP+T+fOaepot/ mMGFdYID4j6FQHz1q9EiUclgd3B5u4CcWpbpsua/sFwbWlM9LG+vU+NcXnAArHiW gxR7P+3FeKGua0MD6WMcpzg4NiN0OTCfdEZgzIpyhxpJy8p020Hpwr1GQ4lMHApX mBmOfMVDq4jVzv539aFl/rxsgAIb99ylyRWHIBP7ARe8MoWq95W+Gh8FmWDPFK47 SRUDXhOU8IpnAUa8YD5r+32q5dXY4M7v3scqSBh4lxni9Em47+goQTcrP5lNXqXl lBTECCyQP8qfytBERyiDVn+jPTlZQR8gBy8sezeTQ9CFah1jbww= =n77z -----END PGP SIGNATURE----- --------------xVK0XLy9X0vduJNb936CFAdr--