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

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

[Natasha Klaus]

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

[PATCH 1/2] userns: reject uid_map/gid_map values that do not fit in u32

[Natasha Klaus]

map_write() parses each line written to uid_map, gid_map or projid_map
with simple_strtoul() and stores the results in the u32 fields of
struct uid_gid_extent. On 64-bit kernels a value above U32_MAX is
silently truncated modulo 2^32. The coding assistant flagged this
truncation site while reading map_write(), and I confirmed it by hand
on a running kernel, where writing to the uid_map of a fresh user
namespace installs a different mapping than the one requested:

  "4294967296 1000 1"  ->  installed as "0 1000 1"
  "4294967297 1000 1"  ->  installed as "1 1000 1"
  "4294967301 1000 1"  ->  installed as "5 1000 1"

The same happens to the lower_first and count fields. Only a literal of
2^64 or more is rejected, and only because simple_strtoul() saturates
to ULLONG_MAX, which truncates onto the (u32)-1 value map_write()
already refuses.

The write succeeds, so a runtime that computes ranges and gets one
wrong ends up with a different map than it asked for and no error to
tell it so.

Parse each field with simple_strtoull() into unsigned long long locals
and fail the write with -EINVAL if any of them exceeds U32_MAX, before
the existing checks, which are kept unchanged and in the same order.
On 32-bit kernels unsigned long is 32 bits wide and the truncation
happens inside simple_strtoul() itself, so a range check on its return
value would never fire there; that is why the values are parsed as
unsigned long long. simple_strtoull() keeps the end pointer behaviour
the multi-line parser relies on.

This is hardening, not a security fix: the truncated values are valid
ids and every later check still applies to them.

Signed-off-by: Natasha Klaus <natalie.klaus@runtimeverification.com>
Assisted-by: LLM
---
checkpatch warns that simple_strtoull() is obsolete. kstrtoull() does not
fit here: the buffer holds several lines of three space-separated fields,
and the parser relies on the end pointer to step from one field to the
next, which kstrto*() does not provide.

 kernel/user_namespace.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 0bed462e9b2a..e7ed475c3685 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -940,6 +940,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	struct uid_gid_map new_map;
 	unsigned idx;
 	struct uid_gid_extent extent;
+	unsigned long long first, lower_first, cnt;
 	char *kbuf, *pos, *next_line;
 	ssize_t ret;
 
@@ -1001,17 +1002,17 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 		}
 
 		pos = skip_spaces(pos);
-		extent.first = simple_strtoul(pos, &pos, 10);
+		first = simple_strtoull(pos, &pos, 10);
 		if (!isspace(*pos))
 			goto out;
 
 		pos = skip_spaces(pos);
-		extent.lower_first = simple_strtoul(pos, &pos, 10);
+		lower_first = simple_strtoull(pos, &pos, 10);
 		if (!isspace(*pos))
 			goto out;
 
 		pos = skip_spaces(pos);
-		extent.count = simple_strtoul(pos, &pos, 10);
+		cnt = simple_strtoull(pos, &pos, 10);
 		if (*pos && !isspace(*pos))
 			goto out;
 
@@ -1020,6 +1021,13 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 		if (*pos != '\0')
 			goto out;
 
+		/* Verify the values fit in the u32 fields of the extent */
+		if (first > U32_MAX || lower_first > U32_MAX || cnt > U32_MAX)
+			goto out;
+		extent.first = first;
+		extent.lower_first = lower_first;
+		extent.count = cnt;
+
 		/* Verify we have been given valid starting values */
 		if ((extent.first == (u32) -1) ||
 		    (extent.lower_first == (u32) -1))
-- 
2.34.1

[PATCH 2/2] selftests/namespaces: add uid_map/gid_map range tests

[Natasha Klaus]

Check that a uid_map or gid_map line with a value that does not fit in
u32 is rejected with EINVAL and installs nothing, for each of the three
fields, and that an in-range line is still accepted and reads back
unchanged.

Signed-off-by: Natasha Klaus <natalie.klaus@runtimeverification.com>
Assisted-by: LLM
---
 tools/testing/selftests/namespaces/.gitignore |   1 +
 tools/testing/selftests/namespaces/Makefile   |   3 +-
 .../selftests/namespaces/uid_map_range_test.c | 155 ++++++++++++++++++
 3 files changed, 158 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/namespaces/uid_map_range_test.c

diff --git a/tools/testing/selftests/namespaces/.gitignore b/tools/testing/selftests/namespaces/.gitignore
index 0989e80da457..995b082357fa 100644
--- a/tools/testing/selftests/namespaces/.gitignore
+++ b/tools/testing/selftests/namespaces/.gitignore
@@ -10,3 +10,4 @@ cred_change_test
 stress_test
 listns_pagination_bug
 regression_pidfd_setns_test
+uid_map_range_test
diff --git a/tools/testing/selftests/namespaces/Makefile b/tools/testing/selftests/namespaces/Makefile
index fbb821652c17..9b22d12708e1 100644
--- a/tools/testing/selftests/namespaces/Makefile
+++ b/tools/testing/selftests/namespaces/Makefile
@@ -13,7 +13,8 @@ TEST_GEN_PROGS := nsid_test \
 		  cred_change_test \
 		  stress_test \
 		  listns_pagination_bug \
-		  regression_pidfd_setns_test
+		  regression_pidfd_setns_test \
+		  uid_map_range_test
 
 include ../lib.mk
 
diff --git a/tools/testing/selftests/namespaces/uid_map_range_test.c b/tools/testing/selftests/namespaces/uid_map_range_test.c
new file mode 100644
index 000000000000..9da02005a09c
--- /dev/null
+++ b/tools/testing/selftests/namespaces/uid_map_range_test.c
@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+
+/*
+ * The fields of a uid_map/gid_map line are stored in u32. Values that
+ * do not fit must be rejected with EINVAL instead of being silently
+ * truncated modulo 2^32, and in-range values must still be accepted.
+ */
+
+static int write_file(const char *path, const char *buf)
+{
+	ssize_t len = strlen(buf);
+	ssize_t ret;
+	int fd;
+
+	fd = open(path, O_WRONLY | O_CLOEXEC);
+	if (fd < 0)
+		return -errno;
+	ret = write(fd, buf, len);
+	if (ret < 0)
+		ret = -errno;
+	close(fd);
+	if (ret < 0)
+		return ret;
+	return ret == len ? 0 : -EIO;
+}
+
+static ssize_t read_file(const char *path, char *buf, size_t size)
+{
+	ssize_t ret;
+	int fd;
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0)
+		return -errno;
+	ret = read(fd, buf, size - 1);
+	if (ret < 0)
+		ret = -errno;
+	else
+		buf[ret] = '\0';
+	close(fd);
+	return ret;
+}
+
+/* Enter a fresh user namespace, remembering the outer ids. */
+static int enter_userns(uid_t *uid, gid_t *gid)
+{
+	*uid = geteuid();
+	*gid = getegid();
+	if (unshare(CLONE_NEWUSER) < 0)
+		return -errno;
+	/* Required before an unprivileged gid_map write. */
+	return write_file("/proc/self/setgroups", "deny");
+}
+
+static void expect_rejected(struct __test_metadata *_metadata,
+			    const char *path, const char *line)
+{
+	char buf[256];
+
+	TH_LOG("writing \"%s\" to %s", line, path);
+	EXPECT_EQ(write_file(path, line), -EINVAL);
+	/* Nothing may have been installed. */
+	EXPECT_EQ(read_file(path, buf, sizeof(buf)), 0) {
+		TH_LOG("installed: %s", buf);
+	}
+}
+
+TEST(uid_map_first_above_u32)
+{
+	char line[64];
+	uid_t uid;
+	gid_t gid;
+	int ret;
+
+	ret = enter_userns(&uid, &gid);
+	if (ret)
+		SKIP(return, "cannot set up user namespace: %s", strerror(-ret));
+	snprintf(line, sizeof(line), "4294967301 %u 1", uid);
+	expect_rejected(_metadata, "/proc/self/uid_map", line);
+}
+
+TEST(uid_map_lower_first_above_u32)
+{
+	char line[64];
+	uid_t uid;
+	gid_t gid;
+	int ret;
+
+	ret = enter_userns(&uid, &gid);
+	if (ret)
+		SKIP(return, "cannot set up user namespace: %s", strerror(-ret));
+	snprintf(line, sizeof(line), "0 %llu 1", (1ULL << 32) + uid);
+	expect_rejected(_metadata, "/proc/self/uid_map", line);
+}
+
+TEST(uid_map_count_above_u32)
+{
+	char line[64];
+	uid_t uid;
+	gid_t gid;
+	int ret;
+
+	ret = enter_userns(&uid, &gid);
+	if (ret)
+		SKIP(return, "cannot set up user namespace: %s", strerror(-ret));
+	snprintf(line, sizeof(line), "0 %u 4294967297", uid);
+	expect_rejected(_metadata, "/proc/self/uid_map", line);
+}
+
+TEST(gid_map_first_above_u32)
+{
+	char line[64];
+	uid_t uid;
+	gid_t gid;
+	int ret;
+
+	ret = enter_userns(&uid, &gid);
+	if (ret)
+		SKIP(return, "cannot set up user namespace: %s", strerror(-ret));
+	snprintf(line, sizeof(line), "4294967301 %u 1", gid);
+	expect_rejected(_metadata, "/proc/self/gid_map", line);
+}
+
+TEST(uid_map_in_range)
+{
+	unsigned int first, lower_first, count;
+	char line[64], buf[256];
+	uid_t uid;
+	gid_t gid;
+	int ret;
+
+	ret = enter_userns(&uid, &gid);
+	if (ret)
+		SKIP(return, "cannot set up user namespace: %s", strerror(-ret));
+	snprintf(line, sizeof(line), "5 %u 1", uid);
+	ASSERT_EQ(write_file("/proc/self/uid_map", line), 0);
+
+	ASSERT_GT(read_file("/proc/self/uid_map", buf, sizeof(buf)), 0);
+	ASSERT_EQ(sscanf(buf, "%u %u %u", &first, &lower_first, &count), 3);
+	EXPECT_EQ(first, 5U);
+	EXPECT_EQ(lower_first, uid);
+	EXPECT_EQ(count, 1U);
+	/* Exactly one extent. */
+	EXPECT_EQ(strchr(buf, '\n'), buf + strlen(buf) - 1);
+}
+
+TEST_HARNESS_MAIN
-- 
2.34.1

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

[Eric W. Biederman]

Natasha Klaus <natalie.klaus@runtimeverification.com> writes:

> 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.

Well error in more cases.  *Shrug* I don't think that even really
hardens anything but it is moderately more correct.

> 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.

Storing an unsigned long into an unsigned long long instead of an
unsigned is a mistake in the opposite direction and is more costly.

A more targeted fix would be to replace simple_strtoul with
kstrtou32.

As for tests you probably also want to test some negative values as
well, and probably a few good values.

Eric

>
> 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