From 63981fc786daaa626cb14d9be1406f674d79f98f Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Sun, 2 Aug 2026 20:00:43 +0200 Subject: [PATCH 1/4] ovl: don't warn when the mount is completed from another user namespace 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+ Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-1-4e987911a39a@kernel.org Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) --- 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); From 79fdf39f1a31f88cb3833b6f8091fbf6acdca2c6 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Sun, 2 Aug 2026 20:00:44 +0200 Subject: [PATCH 2/4] binfmt_misc: don't warn when the mount is completed from another user namespace 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+ Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-2-4e987911a39a@kernel.org Reviewed-by: Jan Kara Signed-off-by: Christian Brauner (Amutable) --- 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. */ From d64ba78b749ad6bae0e9a6f40614eda1c7db2f04 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Sun, 2 Aug 2026 20:00:45 +0200 Subject: [PATCH 3/4] selftests/filesystems: test completing a context from another user namespace 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. Link: https://patch.msgid.link/20260802-work-fill_super-warn-v1-3-4e987911a39a@kernel.org Signed-off-by: Christian Brauner (Amutable) --- tools/testing/selftests/Makefile | 1 + .../filesystems/fscontext_ns/Makefile | 10 + .../fscontext_ns/fscontext_ns_test.c | 239 ++++++++++++++++++ 3 files changed, 250 insertions(+) create mode 100644 tools/testing/selftests/filesystems/fscontext_ns/Makefile create mode 100644 tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c 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 + * + * 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 +#include +#include +#include +#include +#include +#include +#include + +#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 From b64a9f67e082e04835ddd69d422a25168d69375b Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?J=C3=A9r=C3=A9my=20Jean?= Date: Tue, 11 Aug 2026 19:10:11 +0000 Subject: [PATCH 4/4] pid: reject allocations through dead ancestor pid namespaces MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit alloc_pid() checks PIDNS_ADDING only on the leaf pid namespace before making a new struct pid visible in every ancestor namespace. That is insufficient when an unborn descendant pid namespace outlives an ancestor whose init task has already exited. The descendant can still be initialized later through setns(), and the new pid is then published into the dead ancestor as well. Keep the existing ENOMEM behavior, but require PIDNS_ADDING to be set in every namespace that will receive the new pid before publishing any of them. This preserves the invariant that free_pid() never decrements pid_allocated in a namespace whose child_reaper is no longer live. Fixes: a3bdc23ba8ea ("pid_namespace: allow opening pid_for_children before init was created") Signed-off-by: Jérémy Jean Reviewed-by: Pavel Tikhomirov Signed-off-by: Christian Brauner (Amutable) --- kernel/pid.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/pid.c b/kernel/pid.c index f55189a3d07d..d01d0dd7114b 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -324,8 +324,10 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid, * error path may try to wakeup the possibly freed ns->child_reaper. */ retval = -ENOMEM; - if (unlikely(!(ns->pid_allocated & PIDNS_ADDING))) - goto out_free; + for (upid = pid->numbers + ns->level; upid >= pid->numbers; --upid) + if (unlikely(!(upid->ns->pid_allocated & PIDNS_ADDING))) + goto out_free; + for (upid = pid->numbers + ns->level; upid >= pid->numbers; --upid) { /* Make the PID visible to find_pid_ns. */ idr_replace(&upid->ns->idr, pid, upid->nr);