* [PATCH 1/3] ovl: don't warn when the mount is completed from another user namespace
2026-08-02 18:00 [PATCH 0/3] fs: don't warn when a mount is completed from another user namespace Christian Brauner
@ 2026-08-02 18:00 ` Christian Brauner
2026-08-03 12:05 ` Jan Kara
2026-08-02 18:00 ` [PATCH 2/3] binfmt_misc: " Christian Brauner
2026-08-02 18:00 ` [PATCH 3/3] selftests/filesystems: test completing a context " Christian Brauner
2 siblings, 1 reply; 6+ messages in thread
From: Christian Brauner @ 2026-08-02 18:00 UTC (permalink / raw)
To: Amir Goldstein
Cc: Miklos Szeredi, linux-unionfs, Alexander Viro, Jan Kara,
Kees Cook, Laurent Vivier, linux-fsdevel, linux-mm,
Christian Brauner (Amutable), stable
fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor. Nothing ties the task that calls
fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The
fd is inherited across fork() and exec() and it can be passed over a
unix socket.
Completing a context from another user namespace is allowed on purpose.
vfs_cmd_create() authorizes the create with mount_capable(), which for
FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that
succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns.
So an unprivileged task can reach the WARN_ON() in ovl_fill_super():
create a user and a mount namespace in a child, call fsopen("overlay")
there, send the fscontext fd to the parent and let the parent issue
FSCONFIG_CMD_CREATE. Both namespaces come from a plain unshare(1) and no
capability is needed anywhere:
WARNING: fs/overlayfs/super.c:1551 at ovl_fill_super+0x7b9/0x1e20 [overlay]
CPU: 3 UID: 1000 PID: 3243376 Comm: fswarn
Call Trace:
get_tree_nodev+0x71/0xa0
ovl_get_tree+0x15/0x20 [overlay]
vfs_get_tree+0x2a/0x100
vfs_cmd_create+0x60/0xf0
__do_sys_fsconfig+0x4b2/0x500
The child needs the mount namespace because fsopen() itself gates on
may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning
the caller's mount namespace. fsconfig() doesn't repeat that check.
It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be
raised in a loop to taint the kernel and flood the log, and it panics a
kernel booted with panic_on_warn.
Keep refusing the mount and stop warning about it. ovl_parse_param()
already spells a user namespace check this way for Opt_override_creds.
Fixes: 1784fbc2ed9c ("ovl: port to new mount api")
Cc: stable@vger.kernel.org # v6.5+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
fs/overlayfs/super.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 60f0b7ceef0a..60b808b85fc4 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1544,7 +1544,8 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
int err;
err = -EIO;
- if (WARN_ON(fc->user_ns != current_user_ns()))
+ /* The fscontext fd may have been passed to another user namespace. */
+ if (fc->user_ns != current_user_ns())
goto out_err;
ovl_set_d_op(sb);
--
2.53.0
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH 2/3] binfmt_misc: don't warn when the mount is completed from another user namespace
2026-08-02 18:00 [PATCH 0/3] fs: don't warn when a mount is completed from another user namespace Christian Brauner
2026-08-02 18:00 ` [PATCH 1/3] ovl: don't warn when the " Christian Brauner
@ 2026-08-02 18:00 ` Christian Brauner
2026-08-03 12:05 ` Jan Kara
2026-08-02 18:00 ` [PATCH 3/3] selftests/filesystems: test completing a context " Christian Brauner
2 siblings, 1 reply; 6+ messages in thread
From: Christian Brauner @ 2026-08-02 18:00 UTC (permalink / raw)
To: Amir Goldstein
Cc: Miklos Szeredi, linux-unionfs, Alexander Viro, Jan Kara,
Kees Cook, Laurent Vivier, linux-fsdevel, linux-mm,
Christian Brauner (Amutable), stable
fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor. Nothing ties the task that calls
fsconfig(FSCONFIG_CMD_CREATE) to the task that created the context. The
fd is inherited across fork() and exec() and it can be passed over a
unix socket.
Completing a context from another user namespace is allowed on purpose.
vfs_cmd_create() authorizes the create with mount_capable(), which for
FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN), and that
succeeds for a task holding CAP_SYS_ADMIN in an ancestor of fc->user_ns.
So an unprivileged task can reach the WARN_ON() in bm_fill_super():
create a user and a mount namespace in a child, call
fsopen("binfmt_misc") there, send the fscontext fd to the parent and let
the parent issue FSCONFIG_CMD_CREATE. Both namespaces come from a plain
unshare(1) and no capability is needed anywhere:
WARNING: fs/binfmt_misc.c:938 at bm_fill_super+0xa2/0xc0 [binfmt_misc]
CPU: 15 UID: 1000 PID: 3243382 Comm: fswarn
Call Trace:
get_tree_keyed+0x7d/0xb0
bm_get_tree+0x34/0x90 [binfmt_misc]
vfs_get_tree+0x2a/0x100
vfs_cmd_create+0x60/0xf0
__do_sys_fsconfig+0x4b2/0x500
The child needs the mount namespace because fsopen() itself gates on
may_mount(), which asks for CAP_SYS_ADMIN in the user namespace owning
the caller's mount namespace. fsconfig() doesn't repeat that check.
It is a WARN_ON() and not a WARN_ON_ONCE(), so the condition can be
raised in a loop to taint the kernel and flood the log, and it panics a
kernel booted with panic_on_warn.
Keep refusing the mount and stop warning about it. Nothing in
bm_fill_super() depends on the two namespaces matching, it derives
everything from sb->s_user_ns.
Fixes: 21ca59b365c0 ("binfmt_misc: enable sandboxed mounts")
Cc: stable@vger.kernel.org # v6.7+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
fs/binfmt_misc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index c97f10b48b5b..613dd28e3f1a 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -937,7 +937,8 @@ static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
/* last one */ {""}
};
- if (WARN_ON(user_ns != current_user_ns()))
+ /* The fscontext fd may have been passed to another user namespace. */
+ if (user_ns != current_user_ns())
return -EINVAL;
/* Never exec off this instance and never let anything stack on it. */
--
2.53.0
^ permalink raw reply related [flat|nested] 6+ messages in thread* [PATCH 3/3] selftests/filesystems: test completing a context from another user namespace
2026-08-02 18:00 [PATCH 0/3] fs: don't warn when a mount is completed from another user namespace Christian Brauner
2026-08-02 18:00 ` [PATCH 1/3] ovl: don't warn when the " Christian Brauner
2026-08-02 18:00 ` [PATCH 2/3] binfmt_misc: " Christian Brauner
@ 2026-08-02 18:00 ` Christian Brauner
2 siblings, 0 replies; 6+ messages in thread
From: Christian Brauner @ 2026-08-02 18:00 UTC (permalink / raw)
To: Amir Goldstein
Cc: Miklos Szeredi, linux-unionfs, Alexander Viro, Jan Kara,
Kees Cook, Laurent Vivier, linux-fsdevel, linux-mm,
Christian Brauner (Amutable)
fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor, so the task that issues
FSCONFIG_CMD_CREATE need not be the one that created the context.
mount_capable() authorizes that for a caller holding CAP_SYS_ADMIN in an
ancestor of fc->user_ns, which any unprivileged user has over a user
namespace it just created.
binfmt_misc and overlayfs used to WARN_ON() the mismatch. Add a test for
both. Also cover the handover within one user namespace. That is a
supported thing to do and has to keep working.
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
tools/testing/selftests/Makefile | 1 +
.../selftests/filesystems/fscontext_ns/Makefile | 10 +
.../filesystems/fscontext_ns/fscontext_ns_test.c | 239 +++++++++++++++++++++
3 files changed, 250 insertions(+)
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 8d4db2241cc2..b622052ec3e9 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -42,6 +42,7 @@ TARGETS += filesystems/fuse
TARGETS += filesystems/move_mount
TARGETS += filesystems/empty_mntns
TARGETS += filesystems/fsmount_ns
+TARGETS += filesystems/fscontext_ns
TARGETS += firmware
TARGETS += fpu
TARGETS += ftrace
diff --git a/tools/testing/selftests/filesystems/fscontext_ns/Makefile b/tools/testing/selftests/filesystems/fscontext_ns/Makefile
new file mode 100644
index 000000000000..7e3506294757
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fscontext_ns/Makefile
@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_GEN_PROGS := fscontext_ns_test
+
+CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+LDLIBS := -lcap
+
+include ../../lib.mk
+
+$(OUTPUT)/fscontext_ns_test: fscontext_ns_test.c ../utils.c
+ $(CC) $(CFLAGS) -o $@ $^ $(LDLIBS)
diff --git a/tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c b/tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c
new file mode 100644
index 000000000000..0f30a3e6e197
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c
@@ -0,0 +1,239 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2026 Christian Brauner <brauner@kernel.org>
+ *
+ * Test that completing a filesystem context from another user namespace
+ * doesn't warn.
+ *
+ * fsopen() records the caller's user namespace in fc->user_ns and hands
+ * back an ordinary file descriptor. The task that issues
+ * FSCONFIG_CMD_CREATE need not be the one that created the context: the fd
+ * is inherited across fork() and exec() and it can be passed over a unix
+ * socket. vfs_cmd_create() authorizes the create with mount_capable(),
+ * which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN),
+ * and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of
+ * fc->user_ns.
+ *
+ * binfmt_misc and overlayfs used to WARN_ON() that mismatch, which let an
+ * unprivileged user taint the kernel, flood the log and panic a kernel
+ * booted with panic_on_warn. The mount must still be refused, but it must
+ * not warn.
+ */
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "../wrappers.h"
+#include "../utils.h"
+#include "../../kselftest_harness.h"
+
+#ifndef FSCONFIG_CMD_CREATE
+#define FSCONFIG_CMD_CREATE 6
+#endif
+
+/* TAINT_WARN, i.e. bit 9 of /proc/sys/kernel/tainted. */
+#define TAINT_WARN_BIT 9
+
+static bool taint_warn_set(void)
+{
+ unsigned long taint = 0;
+ FILE *f;
+
+ f = fopen("/proc/sys/kernel/tainted", "r");
+ if (!f)
+ return false;
+ if (fscanf(f, "%lu", &taint) != 1)
+ taint = 0;
+ fclose(f);
+
+ return taint & (1UL << TAINT_WARN_BIT);
+}
+
+static int send_fd(int sock, int fd)
+{
+ char cmsgbuf[CMSG_SPACE(sizeof(int))] = {};
+ char b[1] = { 'x' };
+ struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) };
+ struct msghdr msg = {
+ .msg_iov = &iov,
+ .msg_iovlen = 1,
+ .msg_control = cmsgbuf,
+ .msg_controllen = sizeof(cmsgbuf),
+ };
+ struct cmsghdr *cmsg;
+
+ cmsg = CMSG_FIRSTHDR(&msg);
+ cmsg->cmsg_level = SOL_SOCKET;
+ cmsg->cmsg_type = SCM_RIGHTS;
+ cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+ memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
+
+ return sendmsg(sock, &msg, 0) < 0 ? -1 : 0;
+}
+
+static int recv_fd(int sock)
+{
+ char cmsgbuf[CMSG_SPACE(sizeof(int))] = {};
+ char b[1];
+ struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) };
+ struct msghdr msg = {
+ .msg_iov = &iov,
+ .msg_iovlen = 1,
+ .msg_control = cmsgbuf,
+ .msg_controllen = sizeof(cmsgbuf),
+ };
+ struct cmsghdr *cmsg;
+ int fd = -1;
+
+ if (recvmsg(sock, &msg, 0) <= 0)
+ return -1;
+
+ cmsg = CMSG_FIRSTHDR(&msg);
+ if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS)
+ return -1;
+ memcpy(&fd, CMSG_DATA(cmsg), sizeof(int));
+
+ return fd;
+}
+
+/*
+ * Create a context for @fsname in a child and complete it here. With @nest
+ * the child first creates its own user namespace, so that the context is
+ * created in a descendant of the namespace completing it. The child needs a
+ * mount namespace of its own as well: fsopen() gates on may_mount(), which
+ * asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount
+ * namespace.
+ *
+ * Returns the result of FSCONFIG_CMD_CREATE with errno set, or -ENODATA if
+ * the child could not create the context at all.
+ */
+static int create_from_child(const char *fsname, bool nest)
+{
+ int sock[2], fd, ret, status;
+ pid_t pid;
+
+ if (socketpair(AF_UNIX, SOCK_STREAM, 0, sock))
+ return -ENODATA;
+
+ pid = fork();
+ if (pid < 0) {
+ close(sock[0]);
+ close(sock[1]);
+ return -ENODATA;
+ }
+
+ if (pid == 0) {
+ close(sock[0]);
+
+ if (nest && unshare(CLONE_NEWUSER | CLONE_NEWNS))
+ _exit(1);
+
+ fd = sys_fsopen(fsname, 0);
+ if (fd < 0)
+ _exit(1);
+ if (send_fd(sock[1], fd))
+ _exit(1);
+ _exit(0);
+ }
+
+ close(sock[1]);
+ fd = recv_fd(sock[0]);
+ close(sock[0]);
+ wait_for_pid(pid);
+ waitpid(pid, &status, WNOHANG);
+
+ if (fd < 0)
+ return -ENODATA;
+
+ errno = 0;
+ ret = sys_fsconfig(fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+ status = errno;
+ close(fd);
+ errno = status;
+
+ return ret;
+}
+
+FIXTURE(fscontext_ns) {
+ bool warn_before;
+};
+
+FIXTURE_SETUP(fscontext_ns)
+{
+ self->warn_before = taint_warn_set();
+
+ if (setup_userns() != 0)
+ SKIP(return, "setup_userns failed");
+}
+
+FIXTURE_TEARDOWN(fscontext_ns)
+{
+}
+
+/*
+ * The condition the kernel used to WARN about. It has to be refused, and it
+ * has to be refused quietly: an unprivileged task reaches this.
+ */
+FIXTURE_VARIANT(fscontext_ns) {
+ const char *fsname;
+ int expected_errno;
+};
+
+FIXTURE_VARIANT_ADD(fscontext_ns, binfmt_misc) {
+ .fsname = "binfmt_misc",
+ .expected_errno = EINVAL,
+};
+
+FIXTURE_VARIANT_ADD(fscontext_ns, overlay) {
+ .fsname = "overlay",
+ .expected_errno = EIO,
+};
+
+TEST_F(fscontext_ns, create_from_descendant_userns)
+{
+ int ret;
+
+ ret = create_from_child(variant->fsname, true);
+ if (ret == -ENODATA)
+ SKIP(return, "%s unavailable", variant->fsname);
+
+ ASSERT_EQ(-1, ret);
+ ASSERT_EQ(variant->expected_errno, errno);
+
+ /*
+ * Only meaningful if nothing had warned before us. Note that an
+ * unrelated warning racing this test would look like a failure.
+ */
+ if (self->warn_before)
+ TH_LOG("TAINT_WARN already set, not checking for a new warning");
+ else
+ ASSERT_FALSE(taint_warn_set());
+}
+
+/*
+ * The same handover within one user namespace is a supported thing to do and
+ * has to keep working. binfmt_misc takes no options, so the create succeeds
+ * outright and this also shows the test really drives the create path.
+ */
+TEST(create_from_same_userns)
+{
+ int ret;
+
+ if (setup_userns() != 0)
+ SKIP(return, "setup_userns failed");
+
+ ret = create_from_child("binfmt_misc", false);
+ if (ret == -ENODATA)
+ SKIP(return, "binfmt_misc unavailable");
+
+ ASSERT_EQ(0, ret);
+}
+
+TEST_HARNESS_MAIN
--
2.53.0
^ permalink raw reply related [flat|nested] 6+ messages in thread