nix::unistd::getgrouplist buffer overflow (#1060)

* nix::unistd::getgrouplist buffer overflow

* add `unaffected`

* add patched versions

* add affected OSs

* drop severity down to a warning

* note that this requires root to exploit

Co-authored-by: Sergey "Shnatsel" Davidoff <shnatsel@gmail.com>

crates/nix/RUSTSEC-0000-0000.md

@@ -0,0 +1,39 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "nix"
+date = "2021-09-27"
+url = "https://github.com/nix-rust/nix/issues/1541"
+categories = ["memory-corruption"]
+keywords = ["nss"]
+informational = "unsound"
+
+[versions]
+patched = ["^0.20.2", "^0.21.2", "^0.22.2", ">= 0.23.0",]
+unaffected = ["< 0.16.0"]
+
+[affected]
+os = ["linux", "freebsd", "android", "netbsd", "dragonfly", "openbsd", "fuchsia"]
+functions = { "nix::unistd::getgrouplist" = [">= 0.16.0"] }
+```
+
+# Out-of-bounds write in nix::unistd::getgrouplist
+
+On certain platforms, if a user has more than 16 groups, the
+`nix::unistd::getgrouplist` function will call the libc `getgrouplist`
+function with a length parameter greater than the size of the buffer it
+provides, resulting in an out-of-bounds write and memory corruption.
+
+The libc `getgrouplist` function takes an in/out parameter `ngroups`
+specifying the size of the group buffer. When the buffer is too small to
+hold all of the reqested user's group memberships, some libc
+implementations, including glibc and Solaris libc, will modify `ngroups`
+to indicate the actual number of groups for the user, in addition to
+returning an error. The version of `nix::unistd::getgrouplist` in nix
+0.16.0 and up will resize the buffer to twice its size, but will not
+read or modify the `ngroups` variable. Thus, if the user has more than
+twice as many groups as the initial buffer size of 8, the next call to
+`getgrouplist` will then write past the end of the buffer.
+
+The issue would require editing /etc/groups to exploit, which is usually
+only editable by the root user.