Fix memory unsafety in unistd::getgrouplist

Fixes #1541

CHANGELOG.md

@@ -3,6 +3,14 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](https://semver.org/).
 
+## [0.21.2] - 29 September 2021
+### Added
+### Changed
+### Fixed
+
+- Fixed buffer overflow in `unistd::getgrouplist`.
+  (#[1545](https://github.com/nix-rust/nix/pull/1545))
+
 ## [0.21.1] - 13 August 2021
 ### Added
 ### Changed

src/unistd.rs

@@ -1530,8 +1530,7 @@ pub fn getgrouplist(user: &CStr, group: Gid) -> Result<Vec<Gid>> {
         Ok(None) | Err(_) => <c_int>::max_value(),
     };
     use std::cmp::min;
-    let mut ngroups = min(ngroups_max, 8);
-    let mut groups = Vec::<Gid>::with_capacity(ngroups as usize);
+    let mut groups = Vec::<Gid>::with_capacity(min(ngroups_max, 8) as usize);
     cfg_if! {
         if #[cfg(any(target_os = "ios", target_os = "macos"))] {
             type getgrouplist_group_t = c_int;
@@ -1541,6 +1540,7 @@ pub fn getgrouplist(user: &CStr, group: Gid) -> Result<Vec<Gid>> {
     }
     let gid: gid_t = group.into();
     loop {
+        let mut ngroups = groups.capacity() as i32;
         let ret = unsafe {
             libc::getgrouplist(user.as_ptr(),
                                gid as getgrouplist_group_t,