Release 1.11.10: Security Fix for Build User Isolation

# Release 1.11.10 (2017-06-12) This release fixes a security bug in Nix’s “build user” build isolation mechanism. Previously, Nix builders had the ability to create setuid binaries owned by a `nixbld` user. Such a binary could then be used by an attacker to assume a `nixbld` identity and interfere with subsequent builds running under the same UID. To prevent this issue, Nix now disallows builders to create setuid and setgid binaries. On Linux, this is done using a seccomp BPF filter. Note that this imposes a small performance penalty (e.g. 1% when building GNU Hello). Using seccomp, we now also prevent the creation of extended attributes and POSIX ACLs since these cannot be represented in the NAR format and (in the case of POSIX ACLs) allow bypassing regular Nix store permissions. On macOS, the restriction is implemented using the existing sandbox mechanism, which now uses a minimal “allow all except the creation of setuid/setgid binaries” profile when regular sandboxing is disabled. On other platforms, the “build user” mechanism is now disabled. Thanks go to Linus Heckemann for discovering and reporting this bug.

Release 1.11.10 addresses a security vulnerability in Nix's build user isolation that allowed builders to create setuid binaries, potentially enabling attackers to assume a `nixbld` identity. The fix prevents the creation of setuid/setgid binaries using seccomp BPF filters on Linux, a sandbox on macOS, and disabling the build user mechanism on other platforms. The release also restricts the creation of extended attributes and POSIX ACLs. Linus Heckemann is credited with discovering and reporting the vulnerability.