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lix/doc/manual/rl-next/fchmodat2-sandbox.md

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libstore/local-derivation-goal: prohibit creating setuid/setgid binaries With Linux kernel >=6.6 & glibc 2.39 a `fchmodat2(2)` is available that isn't filtered away by the libseccomp sandbox. Being able to use this to bypass that restriction has surprising results for some builds such as lxc[1]: > With kernel ≥6.6 and glibc 2.39, lxc's install phase uses fchmodat2, > which slips through https://github.com/NixOS/nix/blob/9b88e5284608116b7db0dbd3d5dd7a33b90d52d7/src/libstore/build/local-derivation-goal.cc#L1650-L1663. > The fixupPhase then uses fchmodat, which fails. > With older kernel or glibc, setting the suid bit fails in the > install phase, which is not treated as fatal, and then the > fixup phase does not try to set it again. Please note that there are still ways to bypass this sandbox[2] and this is mostly a fix for the breaking builds. This change works by creating a syscall filter for the `fchmodat2` syscall (number 452 on most systems). The problem is that glibc 2.39 is needed to have the correct syscall number available via `__NR_fchmodat2` / `__SNR_fchmodat2`, but this flake is still on nixpkgs 23.11. To have this change everywhere and not dependent on the glibc this package is built against, I added a header "fchmodat2-compat.hh" that sets the syscall number based on the architecture. On most platforms its 452 according to glibc with a few exceptions: $ rg --pcre2 'define __NR_fchmodat2 (?!452)' sysdeps/unix/sysv/linux/x86_64/x32/arch-syscall.h 58:#define __NR_fchmodat2 1073742276 sysdeps/unix/sysv/linux/mips/mips64/n32/arch-syscall.h 67:#define __NR_fchmodat2 6452 sysdeps/unix/sysv/linux/mips/mips64/n64/arch-syscall.h 62:#define __NR_fchmodat2 5452 sysdeps/unix/sysv/linux/mips/mips32/arch-syscall.h 70:#define __NR_fchmodat2 4452 sysdeps/unix/sysv/linux/alpha/arch-syscall.h 59:#define __NR_fchmodat2 562 I added a small regression-test to the setuid integration-test that attempts to set the suid bit on a file using the fchmodat2 syscall. I confirmed that the test fails without the change in local-derivation-goal. Additionally, we require libseccomp 2.5.5 or greater now: as it turns out, libseccomp maintains an internal syscall table and validates each rule against it. This means that when using libseccomp 2.5.4 or older, one may pass `452` as syscall number against it, but since it doesn't exist in the internal structure, `libseccomp` will refuse to create a filter for that. This happens with nixpkgs-23.11, i.e. on stable NixOS and when building Lix against the project's flake. To work around that * a backport of libseccomp 2.5.5 on upstream nixpkgs has been scheduled[3]. * the package now uses libseccomp 2.5.5 on its own already. This is to provide a quick fix since the correct fix for 23.11 is still a staging cycle away. We still need the compat header though since `SCMP_SYS(fchmodat2)` internally transforms this into `__SNR_fchmodat2` which points to `__NR_fchmodat2` from glibc 2.39, so it wouldn't build on glibc 2.38. The updated syscall table from libseccomp 2.5.5 is NOT used for that step, but used later, so we need both, our compat header and their syscall table 🤷 Relevant PRs in CppNix: * https://github.com/NixOS/nix/pull/10591 * https://github.com/NixOS/nix/pull/10501 [1] https://github.com/NixOS/nixpkgs/issues/300635#issuecomment-2031073804 [2] https://github.com/NixOS/nixpkgs/issues/300635#issuecomment-2030844251 [3] https://github.com/NixOS/nixpkgs/pull/306070 (cherry picked from commit ba6804518772e6afb403dd55478365d4b863c854) Change-Id: I6921ab5a363188c6bff617750d00bb517276b7fe
2024-04-14 12:10:23 +00:00
---
synopsis: Creating setuid/setgid binaries with fchmodat2 is now prohibited by the build sandbox
prs: 10501
---
The build sandbox blocks any attempt to create setuid/setgid binaries, but didn't check
for the use of the `fchmodat2` syscall which was introduced in Linux 6.6 and is used by
glibc >=2.39. This is fixed now.
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