[PATCH 0/2] userns: reject out-of-range uid_map/gid_map values

[Natasha Klaus <natalie.klaus@runtimeverification.com>]

map_write() silently truncates uid_map/gid_map/projid_map values above
U32_MAX modulo 2^32 on 64-bit kernels, so e.g. "4294967301 1000 1" is
installed as "5 1000 1" without an error. Patch 1 rejects such values
with -EINVAL. Patch 2 adds a selftest for all three fields and for the
unchanged in-range path.

This is hardening, not a security fix.

The patch and the selftest were drafted by Claude Code (Claude Opus 5.5)
in an interactive session; I reviewed, built and tested them. The
assistant flagged the truncation while reading map_write(), and I
confirmed it by writing to uid_map on a running kernel.

Tested on v7.3-rc4-393-g165768bb7026 under virtme-ng, x86_64 defconfig
with CONFIG_USER_NS: all five selftest cases pass on the patched kernel.
On an unpatched 6.8 host, four of the five fail as expected.

Natasha Klaus (2):
  userns: reject uid_map/gid_map values that do not fit in u32
  selftests/namespaces: add uid_map/gid_map range tests

 kernel/user_namespace.c                       |  14 +-
 tools/testing/selftests/namespaces/.gitignore |   1 +
 tools/testing/selftests/namespaces/Makefile   |   3 +-
 .../selftests/namespaces/uid_map_range_test.c | 155 ++++++++++++++++++
 4 files changed, 169 insertions(+), 4 deletions(-)
 create mode 100644 tools/testing/selftests/namespaces/uid_map_range_test.c


base-commit: 165768bb70265b5c38cf0b73fafd75be235f8b14
-- 
2.34.1