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ba68045187
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 9b88e52846/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
and seccomp 2.5.5 are 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 tested the change by adding the diff below as patch to
`pkgs/tools/package-management/nix/common.nix` & then built a VM from
the following config using my dirty nixpkgs master:
{
vm = { pkgs, ... }: {
virtualisation.writableStore = true;
virtualisation.memorySize = 8192;
virtualisation.diskSize = 12 * 1024;
nix.package = pkgs.nixVersions.nix_2_21;
};
}
The original issue can be triggered via
nix build -L github:nixos/nixpkgs/d6dc19adbda4fd92fe9a332327a8113eaa843894#lxc \
--extra-experimental-features 'nix-command flakes'
however the problem disappears with this patch applied.
Closes #10424
[1] https://github.com/NixOS/nixpkgs/issues/300635#issuecomment-2031073804
[2] https://github.com/NixOS/nixpkgs/issues/300635#issuecomment-2030844251
34 lines
1.3 KiB
C++
34 lines
1.3 KiB
C++
/*
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* Determine the syscall number for `fchmodat2`.
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*
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* On most platforms this is 452. Exceptions can be found on
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* a glibc git checkout via `rg --pcre2 'define __NR_fchmodat2 (?!452)'`.
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*
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* The problem is that glibc 2.39 and libseccomp 2.5.5 are needed to
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* get the syscall number. However, a Nix built against nixpkgs 23.11
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* (glibc 2.38) should still have the issue fixed without depending
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* on the build environment.
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*
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* To achieve that, the macros below try to determine the platform and
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* set the syscall number which is platform-specific, but
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* in most cases 452.
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*
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* TODO: remove this when 23.11 is EOL and the entire (supported) ecosystem
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* is on glibc 2.39.
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*/
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#if HAVE_SECCOMP
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# if defined(__alpha__)
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# define NIX_SYSCALL_FCHMODAT2 562
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# elif defined(__x86_64__) && SIZE_MAX == 0xFFFFFFFF // x32
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# define NIX_SYSCALL_FCHMODAT2 1073742276
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# elif defined(__mips__) && defined(__mips64) && defined(_ABIN64) // mips64/n64
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# define NIX_SYSCALL_FCHMODAT2 5452
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# elif defined(__mips__) && defined(__mips64) && defined(_ABIN32) // mips64/n32
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# define NIX_SYSCALL_FCHMODAT2 6452
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# elif defined(__mips__) && defined(_ABIO32) // mips32
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# define NIX_SYSCALL_FCHMODAT2 4452
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# else
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# define NIX_SYSCALL_FCHMODAT2 452
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# endif
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#endif // HAVE_SECCOMP
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